What Is a CT600? Complete Guide to Your Company Tax Return
Every UK limited company has to file a CT600 u2014 yet it’s one of the least understood forms in the entire tax system, partly because it runs to over 200 boxes and partly because most guidance online buries the basics under jargon. This guide from Filing Accounts UK explains exactly what a CT600 is, what it covers, and how to file it correctly, using only verified facts from official HMRC guidance.
At Filing Accounts, we prepare and file CT600 returns for UK small businesses as part of our annual accounts and Corporation Tax service. Official guidance is available at GOV.UK: Company Tax Returns.
What Is a CT600?
A CT600 is the official Company Tax Return form that every UK limited company uses to report its taxable profits to HMRC and calculate the Corporation Tax due for that accounting period. Think of it as the company equivalent of a personal Self Assessment return u2014 except it reports the company’s finances, not an individual’s.
Although the form itself has over 200 boxes, the vast majority of small companies only ever complete a small fraction of them u2014 the full form is built to handle every possible scenario, from simple trading companies to complex groups with overseas income.
Who Needs to File One?
Every UK limited company must file a CT600 if HMRC has issued a notice to deliver a return u2014 regardless of whether the company made a profit, broke even, or made a loss. This includes:
- Trading companies of any size
- Companies with investment income, even without active trading
- Non-UK companies with a branch or permanent establishment in the UK
- Companies that made a loss u2014 filing is still required, and a loss can often be carried forward or back to reduce tax in another period
The one real exception: a company that has formally notified HMRC it’s dormant, and hasn’t been sent a specific notice to deliver a return, generally doesn’t need to file. See our dedicated guide on whether dormant companies need to file a CT600 for the detail.
What the CT600 Actually Reports
- Company information u2014 basic details and the exact accounting period covered
- Income u2014 trading profits, investment income, property income, and chargeable gains
- Deductions u2014 losses, group relief, and charitable donations
- Tax calculation u2014 the actual Corporation Tax computation, including any marginal relief
- Tax already paid u2014 payments on account or tax deducted at source
What You Submit Alongside the CT600
A complete Company Tax Return submission is actually three things bundled together, all formatted in iXBRL (a tagging format HMRC’s systems can read automatically):
- The CT600 form itself
- Your statutory accounts
- Your tax computations, showing how accounting profit is adjusted to arrive at taxable profit
Manual paper submissions are no longer accepted for the vast majority of companies u2014 filing must be done electronically.
CT600 Deadlines: Two Dates, Not One
This trips up more directors than almost anything else in the Corporation Tax system: the payment deadline and the filing deadline are different dates.
| Deadline | When |
|---|---|
| Corporation Tax payment | 9 months and 1 day after your accounting period ends |
| CT600 filing | 12 months after your accounting period ends |
Example: if your accounting period ends 31 March, your Corporation Tax payment is due by 1 January the following year, but your CT600 itself isn’t due until 31 March u2014 three months later. Most companies calculate their tax and prepare the return together, then pay first and file once everything’s finalised.
What Happens If You File Late
For returns due from 1 April 2026, late filing penalties are:
- £200 immediately once the deadline passes
- A further £200 if the return is still outstanding after 3 months
- Tax-geared penalties on top u2014 10% of any unpaid tax if the return is 6 months late, and a further 10% at 12 months
Interest also accrues on any Corporation Tax paid late, separately from these filing penalties. See our guide on what to do if you can’t pay your Corporation Tax bill if you’re struggling to meet the payment deadline specifically.
Can You Amend a CT600 After Filing?
Yes u2014 you generally have 12 months from the original filing deadline to amend a submitted CT600, which in practice means roughly 2 years from the end of the accounting period itself. Beyond that window, corrections typically require a different process.
How to File: Your Options
- Through an accountant u2014 using HMRC-recognised commercial software on your behalf
- Yourself, via commercial software u2014 since HMRC’s own free online filing service closed to new use on 31 March 2026, this is now the main self-filing route
- A company formation agent u2014 some offer basic filing as part of their service, though usually only for the simplest cases
CT600 at a Glance
| Item | Detail |
|---|---|
| What it is | Company Tax Return, reporting profits and Corporation Tax due |
| Filing deadline | 12 months after accounting period end |
| Payment deadline | 9 months and 1 day after accounting period end |
| Format required | iXBRL, filed electronically |
| Late filing penalty | u00a3200, rising with tax-geared penalties at 6 and 12 months |
| Amendment window | 12 months from the original filing deadline |
Common Mistakes to Avoid
Confusing the Payment and Filing Deadlines
Payment is due 3 months before the return itself u2014 planning around only one of these dates is a common and costly mistake.
Assuming a Loss-Making Year Doesn’t Need Filing
A CT600 is still required even where no tax is owed u2014 and filing on time is what lets you carry losses forward or back correctly.
Leaving It Until the Deadline
With payment due earlier than filing, and iXBRL formatting to get right, preparing early avoids errors and last-minute penalties.
Frequently Asked Questions
Do I need to file a CT600 if my company made no profit?
Yes, if HMRC has issued a notice to deliver a return u2014 filing is required regardless of profit, loss, or nil activity.
What’s the difference between the CT600 payment and filing deadlines?
Payment is due 9 months and 1 day after your accounting period ends; the return itself isn’t due until 12 months after.
Can I still file for free through HMRC?
No u2014 HMRC’s free online CT600 filing service closed on 31 March 2026. Companies now need an accountant or commercial software to file.
Can I amend a CT600 after submitting it?
Yes, generally within 12 months of the original filing deadline.
Need Help With Your CT600? Talk to Filing Accounts UK
At Filing Accounts, we prepare and file CT600 returns for UK small businesses on a fixed fee basis, calculating your Corporation Tax accurately and submitting everything correctly formatted, on time.
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