The Corporation Tax return (CT600): periods, payments and penalties

Introduction to Corporation Tax and the CT600

Corporation Tax is a direct tax levied by HM Revenue and Customs (HMRC) on the profits made by UK-resident companies and certain other organisations. If you run a private limited company, it is legally separate from you as an individual, and its profits are subject to Corporation Tax. This applies to all profits, whether they come from trading, investments, or selling assets (known as chargeable gains). It's crucial to understand that tax is due on profit, not revenue or turnover. You must register your company for Corporation Tax with HMRC within three months of starting business activities, which can include buying, selling, advertising, renting a property, or employing someone. Failure to register in time can lead to penalties.

The Corporation Tax return, formally known as the CT600, is the form your company must complete to declare its tax liability to HMRC. It's not just a simple form; it's a detailed declaration supported by a full set of company accounts and tax computations. The computations show how you've adjusted your accounting profit to arrive at the taxable profit, detailing things like disallowable expenses and capital allowances. Since 2011, almost all companies must file their CT600 online, along with their accounts in a special digital format called iXBRL. This process requires specialist software or the services of an accountant, as HMRC's free tool has been phased out. The CT600 summarises the entire financial period and is the primary tool for ensuring your company is tax-compliant.

Every limited company that is active must file a CT600, even if it makes a loss and has no tax to pay. The only general exception is for 'dormant' companies, which have had no significant accounting transactions during the financial year. Filing a return for a loss-making period is important as it allows the company to claim relief for those losses against past or future profits, potentially resulting in a tax refund or a reduced tax bill later. Understanding your obligations around the CT600 is not optional; it’s a fundamental part of a director's duties. Neglecting these responsibilities can lead to escalating penalties, interest charges, and formal investigations from HMRC, causing significant financial and administrative burdens for your business.

Worth knowing: Filing a CT600 is a legal requirement for all active UK limited companies, used to report profits and calculate the Corporation Tax due to HMRC.

Understanding Your Corporation Tax Accounting Period

Your company's Corporation Tax Accounting Period (CTAP) is the timeframe for which you must calculate and pay Corporation Tax. For most companies, this period aligns with the annual accounts they prepare and file at Companies House. A CTAP cannot be longer than 12 months. When you first incorporate your company, your first accounting period starts on the date of incorporation and typically ends on the 'accounting reference date' (ARD) set by Companies House, which is usually the last day of the month one year after incorporation. For example, if your company is incorporated on 10 October 2025, your first ARD will be 31 October 2026. This creates a first accounting period longer than 12 months, which has specific implications for your tax return.

When a company's first set of accounts covers a period longer than 12 months, it must file two Corporation Tax returns. This is a common point of confusion for new directors. HMRC rules state that a CTAP cannot exceed 12 months. Therefore, you must split the long period into two separate CTAPs. The first CTAP will be for the first 12 months, and the second will be for the remaining days. For a company incorporated on 10 October 2025 with an ARD of 31 October 2026, the first CTAP would be 10 October 2025 to 9 October 2026 (12 months). The second CTAP would be 10 October 2026 to 31 October 2026. You must prepare two CT600s, apportioning your profits and losses appropriately between the two periods.

You can change your company's year-end date (the ARD) by filing a form AA01 with Companies House. This might be done to align with a seasonal business cycle or a parent company's financial year. Changing your ARD can create either a short (less than 12 months) or a long (more than 12 months) accounting period, with the same tax implications as mentioned above for long periods. If you shorten the period, you simply file one CT600 for that shorter period. Accurately determining your CTAPs is the essential first step in the tax filing process, as all your deadlines for payment and filing are calculated from the end date of each period. Getting this wrong is a frequent cause of accidental late filings and penalties.

Calculating Your Corporation Tax Liability

Your company's Corporation Tax bill is calculated on its 'profits chargeable to Corporation Tax', not the accounting profit shown in your statutory accounts. The calculation starts with your pre-tax profit from the income statement, but this figure must be adjusted for tax purposes. This involves 'adding back' any expenses included in the accounts that are not allowable for tax, and deducting any specific tax reliefs or allowances that are not in the accounts. For instance, depreciation of assets is a standard accounting entry, but for tax it is disallowed. Instead, you deduct a different amount known as 'capital allowances'. Similarly, expenses like entertaining clients are not tax-deductible and must be added back to your profit before calculating the tax due.

For the financial year starting 1 April 2025, the Corporation Tax rates depend on your company's profit level. The 'small profits rate' of 19% applies to profits up to £50,000. For profits over £250,000, the 'main rate' of 25% applies. If your profits fall between these two thresholds, you pay tax at the 25% main rate but can claim 'Marginal Relief'. This relief provides a gradual transition between the two rates, preventing a sudden tax jump. The £50,000 and £250,000 thresholds are reduced for companies with accounting periods shorter than 12 months and for those with 'associated companies' (other companies under common control), which can push a company into a higher effective tax rate sooner than expected.

Understanding these adjustments is key to accurate tax computation. Common mistakes include trying to claim for non-business travel, private expenses, or the full cost of a vehicle with private use. Capital allowances are particularly important; the Annual Investment Allowance (AIA) and Full Expensing rules allow for significant upfront tax relief on the purchase of equipment, which can dramatically reduce your taxable profit. Getting professional advice is often essential to ensure all allowable expenses are claimed and all disallowable ones are correctly added back. This not only ensures compliance but also optimises your company's tax position, ensuring you don't overpay.

• Client entertainment costs
• Depreciation charged in the accounts
• Private expenses or non-business elements of mixed-use expenses
• Fines and penalties (e.g., parking fines or HMRC penalties)
• Donations to non-registered charities

How to Complete and File the CT600 Return

Filing your Corporation Tax return is a multi-step digital process. It is no longer possible for most companies to file using a paper form. You must submit the CT600 form, your company’s statutory accounts, and your tax computations to HMRC online. A critical technical requirement is that the accounts must be in iXBRL (inline eXtensible Business Reporting Language) format. This format tags the financial data within your accounts so it can be automatically read and analysed by HMRC's systems. Most standard accounting software packages can produce accounts in this format, but you must ensure the package you use is compatible and recognised by HMRC. Without iXBRL-tagged accounts, your submission will be rejected.

Given the technical filing requirements, you have two main options: use approved third-party commercial software or hire an accountant. HMRC previously offered a free filing tool, but this service was withdrawn for periods ending after 31 March 2021, making professional software or advice almost essential. The software will guide you through completing the various sections of the CT600, which requires details about your company, turnover, profit/loss, tax calculations, and any reliefs being claimed. The software will also handle the iXBRL tagging and the online submission process. If you use an accountant, they will manage this entire process on your behalf, ensuring accuracy and compliance based on the records you provide.

1. 1

   Finalise your company's annual accounts (profit and loss statement, balance sheet) for the accounting period.

2. 2

   Prepare the detailed Corporation Tax computation, starting with accounting profit and making all necessary tax adjustments (e.g., adding back depreciation, deducting capital allowances).

3. 3

   Use compatible commercial software or your accountant's portal to complete all relevant sections of the CT600 form, transferring the figures from your computation.

4. 4

   Ensure your statutory accounts are correctly converted into the mandatory iXBRL format. The software or your accountant will handle this tagging process.

5. 5

   Attach the iXBRL accounts and the tax computations (usually as a PDF) to your completed CT600 form within the software.

6. 6

   Submit the complete package online to HMRC through the Government Gateway before your filing deadline.

7. 7

   Receive an electronic confirmation from HMRC that your return has been successfully submitted and keep this for your records.

Worth knowing: Filing the CT600 is a mandatory online process requiring iXBRL accounts and specialist software or an accountant.

Corporation Tax Deadlines: Filing and Payment

One of the most common and costly mistakes a company director can make is confusing the deadline for filing the Corporation Tax return with the deadline for paying the tax. They are two separate dates, and the payment deadline is almost always earlier. Your deadline to pay your Corporation Tax is 9 months and one day after the end of your Corporation Tax Accounting Period. For a typical company with a 31 December year-end, the tax for the period ending 31 December 2025 would be due on 1 October 2026. You must pay electronically, as HMRC no longer accepts payment by cheque at a post office. Ensure you use the correct 17-character payment reference number for your accounting period to avoid payment allocation errors.

The deadline to file your CT600 return is later: 12 months after the end of your accounting period. For that same company with a 31 December 2025 year-end, the filing deadline would be 31 December 2026. While this gives you more time to prepare the return, the tax liability itself must have been estimated and paid over three months earlier. This structure requires proactive financial management. You cannot wait until you file the return to think about the payment. Even if you plan to file early, the payment deadline remains fixed at 9 months and one day. Missing this payment deadline results in HMRC charging interest on the outstanding amount from the day it was due until the day it is paid in full.

A different set of rules applies to companies with very large profits. If your company’s annual profits are over £1.5 million, you are generally required to pay your Corporation Tax in instalments. These instalments are due during the financial year itself, rather than after it has ended. The £1.5 million threshold is divided by the number of 'associated companies' you have, so groups of companies may fall into this regime even if no single company has profits that high. The instalment payment system is complex, with payments typically due in the 7th, 10th, 13th, and 16th months following the start of the accounting period. Companies in this bracket almost universally rely on professional advice to manage their tax payments correctly.

Penalties for Late Filing and Late Payment

HMRC enforces strict penalties for failing to meet your Corporation Tax obligations, and these are applied separately for late filing and late payment. The late filing penalty regime is automated and unforgiving. A penalty is charged even if your company has no tax to pay or has already paid the tax due. The initial penalty for filing your CT600 just one day late is £100. If the return is still outstanding after three months, a further £100 penalty is issued. If it remains unfiled after six months, HMRC will estimate the tax due and add a 10% penalty on that estimated liability. If the failure continues to 12 months, another 10% penalty is added. These penalties are doubled if a company files late for three consecutive periods.

Late payment, on the other hand, results in interest charges. HMRC charges 'late payment interest' on the amount of unpaid tax from the date it was due until the date of payment. This interest rate is set at the Bank of England base rate plus 2.5%, so it can accumulate quickly. Unlike penalties, interest is designed to compensate the Exchequer for the delay in receiving funds, not to punish. However, if tax remains unpaid for a significant period, separate late payment penalties can also be levied, in addition to the interest. These penalties are a percentage of the unpaid tax and are charged at set trigger points, typically starting after 30 days.

It is crucial to understand that these penalties and interest charges can accumulate into substantial sums, placing a severe financial strain on a business. For example, a company that files six months late and has an estimated tax bill of £20,000 could face £200 in fixed penalties plus a £2,000 tax-geared penalty, on top of any interest for paying late. The only way to avoid these is through diligent planning and timely action. If you anticipate difficulty in paying your tax bill on time, you should contact HMRC's business payment support service before the deadline. They may be able to agree a 'Time to Pay' arrangement, which could prevent late payment penalties, although interest will still be charged.

Worth knowing: HMRC applies separate and cumulative penalties for late filing and late payment, which apply even if no tax is due.

Capital Allowances: A Key Tax Deduction

Capital allowances are a vital form of tax relief for limited companies, allowing you to deduct the cost of certain business assets from your profits over time. They are the tax equivalent of depreciation, which is not an allowable expense for tax purposes. Instead of deducting depreciation, you claim capital allowances. This system covers most assets you buy for your business that you keep to use, known as 'plant and machinery'. This includes items like office equipment, computers, commercial vehicles (vans, lorries), and machinery used in a factory. Understanding how to claim these allowances is essential for reducing your Corporation Tax bill, as they can significantly lower your taxable profits.

The most generous form of relief for many companies is the Annual Investment Allowance (AIA). This allows you to deduct 100% of the cost of qualifying plant and machinery in the year of purchase, up to a very high annual limit (currently £1 million). For most small and medium-sized companies, this means the full cost of equipment can be written off against profit immediately. For companies paying the 25% main rate of Corporation Tax, an even more powerful relief called 'Full Expensing' is available for new and unused main rate plant and machinery. This works like the AIA but has no upper financial limit, providing a huge incentive for large-scale investment. A separate 50% first-year allowance also applies to new special rate assets, such as long-life items or integral building features.

If an asset does not qualify for AIA or Full Expensing, or if you have already used your AIA limit, you can claim relief through Writing Down Allowances (WDA). Assets are pooled together, and you claim a percentage of the pool's value each year. The main rate of WDA is 18% per year, and the special rate is 6% per year. Cars are treated separately and have their own allowance rates based on their CO2 emissions. Maximising capital allowance claims requires careful record-keeping of asset purchases and disposals. It is a complex area where professional advice can provide substantial value, ensuring your company benefits fully from the available reliefs on its investments.

• Computers, printers, and other IT equipment
• Office desks, chairs, and other furniture
• Vans, lorries, and tractors (but not most cars)
• Tools and machinery used for your trade
• Integral features of a building, such as air conditioning or electrical systems
• Alterations to a building to install plant and machinery

Tax Reliefs and Making Use of Losses

Beyond capital allowances, companies can claim various other tax reliefs to reduce their Corporation Tax liability. One of the most significant is relief for trading losses. If your company spends more than it earns in an accounting period, it makes a loss. This loss is a valuable asset that can be used to reclaim tax paid in the past or reduce tax due in the future. The most common way to use a loss is to carry it forward and set it against future profits from the same trade. This is an automatic and effective way to reduce future tax bills. For example, a £20,000 loss in one year can be used to make the first £20,000 of profit in the next year tax-free, subject to certain restrictions for very large profits.

Alternatively, a trading loss can be 'carried back' to be set against total profits of the preceding 12 months. This is a powerful option as it can generate an immediate cash tax refund from HMRC. If your company was profitable last year and paid Corporation Tax, but made a loss this year, you can use this year's loss to reduce last year's profit and claim back the tax you overpaid. This provides a valuable cash flow boost when the business may need it most. The decision to carry a loss back for an immediate refund versus carrying it forward for a future saving depends on your cash flow needs and expectations of future profitability and tax rates.

Companies engaged in innovation may be able to claim Research and Development (R&D) tax relief. This is a generous government incentive that allows companies to deduct an extra percentage of their qualifying R&D costs from their yearly profit, or to claim a payable tax credit if the company is loss-making. Qualifying activities must involve seeking an advance in science or technology. Other reliefs include the Patent Box, which allows a lower rate of Corporation Tax on profits earned from patented inventions, and reliefs for creative industries like film, television, and video games. Investigating these reliefs is crucial, as they can transform a company's tax profile.

• Research & Development (R&D) Tax Reliefs
• The Patent Box regime for profits from patented inventions
• Creative Industry Tax Reliefs (e.g., for film, theatre, video games)
• Relief on goodwill and intangible assets
• Terminal loss relief when a business ceases to trade

Essential Record-Keeping for Corporation Tax

Maintaining complete, accurate, and well-organised records is a legal requirement for all limited companies and the foundation of a compliant Corporation Tax return. HMRC requires you to keep all records that are needed to prepare and support the figures in your CT600. This is not limited to just your final accounts; it includes the underlying documents that prove the transactions. You must be able to show evidence for all income and expenditure, including bank statements, sales invoices, purchase receipts, and expense claims. It also includes records of company assets and debts. The responsibility for ensuring these records are kept falls squarely on the company directors.

The law states that you must keep your company's accounting records for at least six years from the end of the financial year they relate to. For example, for a year-end of 31 December 2025, you must keep the records until at least 31 December 2031. If you file your return late, you must keep the records for longer. Furthermore, if HMRC opens an enquiry into your tax return, you must keep the relevant records until the enquiry is complete, even if that is beyond the normal six-year period. These records can be kept digitally or on paper, but digital record-keeping is strongly recommended for efficiency, security, and ease of access.

Failure to keep adequate records can lead to significant problems. Without proper documentation, it is impossible to accurately calculate your profit and tax liability, which could lead to you overpaying or underpaying tax. If HMRC conducts a compliance check and finds your records are inadequate, they can issue a penalty of up to £3,000. In serious cases of deliberate concealment or destruction of records, criminal prosecution is possible. Using modern cloud accounting software is the best way to meet these obligations. It helps you to capture receipts, track income and expenses in real-time, and provides a clear audit trail to support every figure in your tax return.

Worth knowing: You must keep all business records for at least six years after the end of the financial year, as failure to do so can result in significant penalties.

Common questions

Official sources & further reading

This guide draws on the official guidance below. Rules, fees and individual circumstances can change; check the relevant source before acting.

• GOV.UK - Corporation Tax
• GOV.UK - Company Tax Returns
• GOV.UK - Pay your Corporation Tax bill
• GOV.UK - Corporation Tax: penalties

General UK information, primarily for private companies limited by shares. This is not personalised legal, tax or financial advice. An accountant or solicitor can help apply the rules to your situation.