What Is a CT600 Form? CT600 Online Filing and How to File Without Software

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If you run a limited company in the UK, you will normally need to file a Corporation Tax return with HM Revenue & Customs (HMRC). The main form used for this is called the CT600, but the CT600 is only one part of the complete Company Tax Return.

The rules have also changed recently. Since 1 April 2026, HMRC’s old online service for filing Company Tax Returns has closed, meaning most companies can no longer simply log in to HMRC and complete a CT600 directly on the HMRC website. You now normally need commercial software to submit the return electronically.

What is a CT600 form?

A CT600 is the main form used to submit a company’s Corporation Tax return to HMRC. It contains information about the company, its accounting period, taxable income, deductions, reliefs and the amount of Corporation Tax payable.

However, a CT600 is not the whole Company Tax Return. HMRC explains that a Company Tax Return consists of the CT600, any required supplementary pages, the company’s accounts and tax computations.

This is important because you cannot normally complete a Corporation Tax return simply by taking the profit shown in the company’s accounts and entering that figure into the CT600.

The company’s accounting profit usually needs to be adjusted for Corporation Tax purposes. For example, some expenses shown in the accounts may not be allowable for Corporation Tax, while capital allowances and other tax adjustments may need to be included.

The result is the company’s taxable profit, which is then used to calculate the Corporation Tax liability.

What information is included on a CT600?

The CT600 asks for information about the company and its Corporation Tax position for the relevant accounting period.

Depending on the company’s circumstances, this can include:

• company name and registration details;
• the accounting period covered by the return;
• income and turnover;
• chargeable gains;
• profit before deductions and reliefs;
• deductions and tax reliefs;
• taxable total profits;
• Corporation Tax calculations;
• tax already paid or overpaid;
• the amount of Corporation Tax payable;
• information about certain claims, elections or special circumstances.

There are also supplementary CT600 forms for particular situations. For example, CT600A is used for certain close company loans and benefits provided to participators, while other supplementary pages cover areas such as group relief, controlled foreign companies, research and development and other specialist areas.

You therefore do not necessarily need to complete every section or supplementary page of the CT600. What you need depends on the company’s circumstances.

Is a CT600 the same as company accounts?

No. The CT600 and company accounts are different documents, although they are closely connected.

Company accounts show the financial performance and financial position of the company, while the Corporation Tax return calculates the company’s liability to Corporation Tax.

For example, a company’s accounts might show an accounting profit of £30,000. The taxable profit could be different after making the required Corporation Tax adjustments.

When filing a Company Tax Return, HMRC generally requires the company accounts and tax computations to accompany the CT600. The computations explain how the figures in the tax return have been calculated from the accounts.

This is one reason why preparing a CT600 is more involved than simply filling in a form.

If you are also trying to understand the accounts side of your limited company, you may find our guide What Are Micro Entity Accounts? useful, particularly if your company qualifies for the micro-entity regime: What Are Micro Entity Accounts?

Can I file a CT600 online?

Yes, Corporation Tax returns are normally filed electronically, but the process changed significantly from 1 April 2026.

The previous HMRC online service that allowed companies to prepare and submit their Company Tax Return directly through HMRC closed on 31 March 2026.

From 1 April 2026, companies are generally required to use commercial software to file their Company Tax Return with HMRC. HMRC maintains a list of recognised commercial software products that can be used to submit CT600 returns.

This means that an older guide telling you to log into the HMRC Corporation Tax online service and complete your CT600 there is now out of date.

How do I file a CT600 online in 2026?

The normal process is now to use commercial Corporation Tax software.

The basic process is:

1. Prepare the company’s accounts for the relevant accounting period.
2. Prepare the Corporation Tax computation.
3. Calculate the company’s taxable profit.
4. Calculate the Corporation Tax liability.
5. Enter or import the relevant information into Corporation Tax software.
6. Attach or generate the required accounts and computations in the appropriate electronic format.
7. Complete the CT600 and any required supplementary pages.
8. Check the return carefully.
9. Submit the Company Tax Return electronically to HMRC.
10. Pay the Corporation Tax by the relevant payment deadline.

Some commercial software can also allow you to file the company’s accounts with Companies House, although the Companies House filing and HMRC Corporation Tax filing are separate obligations. GOV.UK confirms that companies file their accounts with Companies House and their Company Tax Return with HMRC.

Can I file CT600 without software?

For most companies, no, not if you mean filing the CT600 online without using commercial software.

This is one of the most important changes introduced in 2026.

The old HMRC online filing service closed on 31 March 2026. From 1 April 2026, HMRC says companies should use commercial software to file their Company Tax Return and accounts with HMRC.

You can still download the CT600 form from GOV.UK, but that does not mean you can simply complete the PDF and email it to HMRC.

The paper CT600 is now generally only available in limited circumstances, such as where the company has a reasonable excuse for being unable to file online or where the company is filing in Welsh.

Therefore, if you are asking “Can I file my CT600 online without buying accounting software?”, the answer under the rules applying from April 2026 is generally no.

Can I file a CT600 myself without an accountant?

Yes. You do not have to use an accountant simply because the return has to be filed using commercial software.

A company director can prepare and submit the Company’s Tax Return themselves if they understand the company’s accounts and Corporation Tax requirements and have suitable software that supports Corporation Tax filing.

The important distinction is that you can do it yourself, but you generally need compatible commercial software to submit it electronically.

The software itself does not replace the need to understand the tax calculations. You are still responsible for making sure the information submitted to HMRC is correct.

For a very straightforward company, this may be manageable. However, Corporation Tax becomes more complicated where the company has areas such as dividends, directors’ loans, capital allowances, losses, associated companies, chargeable gains, research and development claims or transactions involving connected parties.

What do I need before filing a CT600?

Before starting the CT600, you should normally have the company’s accounts and all relevant information needed to calculate the Corporation Tax liability.

This can include:

• the company’s accounting records;
• sales and income information;
• business expenses;
• bank records;
• details of fixed assets and purchases;
• payroll information;
• details of dividends;
• directors’ loan account information;
• previous Corporation Tax losses;
• capital allowance information;
• details of interest and finance costs;
• information about any asset disposals;
• details of any Corporation Tax reliefs or claims;
• the company’s accounting period dates.

You should also make sure you know the company’s Corporation Tax accounting period, because this can be different from the company’s financial year or the period covered by its Companies House accounts.

Do I need to file accounts with the CT600?

Yes, the Company Tax Return normally includes the company’s accounts and tax computations as well as the CT600.

HMRC requires the computations to show how the figures entered on the return have been calculated from the company’s accounts. For online filing, the accounts and computations generally need to be provided in the required electronic format, including iXBRL where applicable.

This is why Corporation Tax software is more than just an electronic version of the CT600 form. The software needs to handle the information and electronic formats required by HMRC.

If your company is dormant, the situation can be different depending on whether HMRC considers the company dormant for Corporation Tax purposes. We explain this in more detail in our article Do I have to file dormant accounts with HMRC?: Do I have to file dormant accounts with HMRC?

When is a CT600 due?

The Company Tax Return is normally due 12 months after the end of the accounting period.

This is different from the Corporation Tax payment deadline.

For most companies, Corporation Tax itself is due 9 months and 1 day after the end of the accounting period.

For example, if a company’s accounting period ends on 31 March 2026, the Corporation Tax would normally be due by 1 January 2027, while the Company Tax Return would normally be due by 31 March 2027.

It is therefore important not to confuse the tax payment deadline with the CT600 filing deadline.

What happens if I file my CT600 late?

If a Company Tax Return is filed late, HMRC can charge a penalty.

The initial penalty is generally £100 for filing up to three months late. Further penalties can apply if the return remains outstanding for longer.

Interest can also apply to unpaid Corporation Tax, so leaving the return or tax payment until the last minute can become expensive.

You should therefore keep track of both deadlines: the date by which Corporation Tax must be paid and the later deadline for filing the Company Tax Return.

Can an accountant file a CT600 for me?

Yes. You can appoint an accountant or tax agent to deal with the Corporation Tax return on behalf of your company.

This can be particularly useful if the company has tax adjustments that are difficult to calculate or if you are unsure whether particular expenses, assets or transactions should be included in the Corporation Tax computation.

An accountant can prepare the accounts, Corporation Tax computation and CT600 and submit the return electronically using suitable Corporation Tax software.

This can also reduce the risk of submitting a return with incorrect tax calculations, missing supplementary pages or incorrectly treated expenses.

If you are looking for help with your company’s year-end accounts as well as the Corporation Tax return, you may also want to read our guide on how much accountants charge for year-end accounts in the UK: How much do accountants charge for year-end accounts in the UK?

Can I still download a CT600 form from HMRC?

Yes. HMRC still publishes the CT600 form and guidance on GOV.UK.

However, downloading the form does not mean that you can normally complete it as a PDF and send it to HMRC instead of filing electronically.

The paper form is mainly available where a company has a reasonable excuse for not filing online or where it is filing in Welsh. HMRC’s current guidance specifically states that commercial software should be used for normal electronic filing.

So, if you have found an old article explaining how to print a CT600, fill it in and post it to HMRC, check the date of the article carefully because the rules changed in 2026.

Is CT600 online filing free?

The old HMRC online service was available without purchasing commercial Corporation Tax software, but that service closed on 31 March 2026.

The current system requires most companies to use commercial software to file their Company Tax Return electronically. HMRC publishes a list of software suppliers whose products can be used for Corporation Tax filing.

Some commercial products may offer free or low-cost options, while others charge a fee for Corporation Tax filing, accounts preparation or both.

The important point is that you should check that the software specifically supports Corporation Tax CT600 filing, rather than assuming that any bookkeeping or accounting software can submit a CT600.

What is the easiest way to file a CT600?

For a straightforward company, the easiest option is usually to use Corporation Tax software that can prepare the accounts, tax computation and CT600 together.

You should first make sure the company’s bookkeeping is complete and that the accounts reconcile to the bank and other accounting records. You can then prepare the year-end accounts, make the required Corporation Tax adjustments, calculate the taxable profit and submit the CT600 electronically.

If you are not comfortable with Corporation Tax calculations, using an accountant may be simpler than trying to work out the tax treatment yourself, particularly where the company has capital assets, losses, directors’ loans or other complicated transactions.

What is the difference between a CT600 and a Self Assessment tax return?

A CT600 is used for a company’s Corporation Tax return, whereas a Self Assessment tax return is normally used by an individual or other taxpayer who has a personal tax reporting obligation.

For example, a limited company might have to file a CT600 with HMRC to report its Corporation Tax liability. The company’s director may separately need to file a personal Self Assessment tax return if they have income that needs to be reported personally.

The two returns should therefore not be confused. The CT600 deals with the company’s tax position, while Self Assessment deals with the individual’s personal tax position.

Can I file my company’s accounts and CT600 together?

Sometimes, yes.

Some commercial software allows a private limited company to prepare and submit its accounts and Company Tax Return together. However, they remain separate filing obligations to two different organisations: Companies House and HMRC.

GOV.UK confirms that some software allows eligible private limited companies to file their accounts and Company Tax Return together.

This can make the process easier, but you should still check that both filings have actually been submitted and accepted.

What should I do if I need to file a CT600 now?

If you need to file a Corporation Tax return now, the first thing to establish is whether the return is already overdue and which accounting period it covers.

You should then prepare the company’s accounts and Corporation Tax computation and use suitable commercial software to submit the CT600 electronically.

If you are a director of a small UK limited company and the Corporation Tax calculation is more complicated than expected, getting professional help can be worthwhile because the CT600 is not simply a form where you enter the company’s accounting profit and let HMRC calculate everything for you.

For companies with straightforward accounts, the process can be relatively simple, but it is still important to make sure the accounting figures, Corporation Tax adjustments and final tax liability are all correct before submitting the return.

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