Do I Need to File a CT600?
If you run a UK limited company, one of the questions you'll face early on is whether you need to file a company tax return — officially known as the CT600. The short answer for most companies is yes, but there are specific circumstances that change what's required.
This guide explains who must file a CT600, who is exempt, and what happens with dormant companies or businesses that haven't started trading yet.
What Is a CT600?
The CT600 is the official HMRC form for a company tax return. It sets out your company's profits (or losses) for an accounting period and calculates how much corporation tax you owe. Every UK limited company and most unincorporated associations are required to file one.
Even if your company made no profit, or has no corporation tax to pay, you may still be legally required to file. The obligation to file is separate from the obligation to pay.
For a full walkthrough of what the form contains, see our company tax return complete guide.
Who Must File a CT600?
HMRC requires you to file a company tax return if you receive a "notice to deliver a Company Tax Return" from HMRC. This notice is typically issued automatically when:
- Your company is incorporated at Companies House
- Your company comes out of a dormant period and starts trading
- HMRC identifies that your company has taxable income or gains
- Made a loss during the period
- Has no corporation tax to pay
- Was inactive for part of the year
- Has already paid any tax due in advance
Limited Companies
All UK private limited companies (Ltd) and public limited companies (PLC) that receive a filing notice from HMRC must file a CT600. This applies equally to micro-entities, small companies, and companies in their first year of trading.
Unincorporated Associations and Clubs
Most unincorporated associations — such as members' clubs, sports clubs, and community groups — are also within the scope of corporation tax and must file if they receive a notice to deliver. Associations that owe less than £100 in tax may be treated as dormant by HMRC, but this must be confirmed formally.
What About Sole Traders and Partnerships?
If you are self-employed as a sole trader, or you are a member of a partnership, you do not file a CT600. Instead, you report your income through a Self Assessment tax return — an SA100 for individuals, or an SA800 for partnerships.
The CT600 applies to companies, not individuals trading in their own name.
Who Does Not Need to File?
Not every business entity must file a CT600. You do not need to file if:
- You are a sole trader or partner (use Self Assessment instead)
- HMRC has confirmed in writing that your company is dormant and does not need to file
- Your company is a registered charity or Community Amateur Sports Club (CASC) that meets the conditions for tax exemption
Dormant Companies: Special Rules
A dormant company is one that has had no significant financial transactions during its accounting period. HMRC and Companies House each have their own definitions of dormancy, and they are not the same.
HMRC's Definition of Dormant
For corporation tax purposes, a company is dormant if it has had no "significant transactions" during the period. HMRC considers the following to potentially qualify:
- A newly formed company that hasn't started trading yet
- A company that has ceased trading with no remaining income sources
- A flat management company with no income
Does a Dormant Company Need to File a CT600?
This depends on whether HMRC has formally confirmed dormancy:
- If HMRC has not yet confirmed dormancy: You must file a CT600 showing that the company is dormant. This is how HMRC registers the position and acknowledges your status.
- Once HMRC confirms dormancy: You no longer need to file annual CT600 returns, provided HMRC does not issue a new notice to deliver.
- If HMRC issues a new notice: Even a previously confirmed dormant company must file again.
Notifying HMRC of Dormancy
To tell HMRC that your company is dormant, you can either:
- File a CT600 for the period showing no activity
- Contact HMRC's Corporation Tax helpline or write to them directly
Important: you still need to file annual accounts with Companies House separately, even if your company is dormant for HMRC purposes. These are entirely separate obligations.
New Companies: When Do You First Need to File?
When you incorporate a new limited company, HMRC is automatically notified by Companies House. HMRC will then write to the company's registered address with a Corporation Tax Reference Number and a notice to deliver a return.
Your first accounting period for corporation tax begins on the date of incorporation. You typically have 12 months from the end of that first accounting period to file your CT600.
Be aware of a few important points:
- If your accounting period is longer than 12 months (which can happen when a company sets an early year-end after incorporation), HMRC will require two separate CT600 returns — one for the first 12 months and one for the remainder.
- You must register for corporation tax within 3 months of starting to trade — failure to notify HMRC can result in penalties.
- HMRC will not always prompt you at exactly the right time. Keep track of your own filing obligations.
Accounting Periods: What You're Filing For
Each CT600 covers a single accounting period — typically 12 months, aligned with your company's financial year. HMRC defines your accounting period based on the dates your company reports to Companies House.
Key points:
- You cannot file a CT600 for a period longer than 12 months — extended periods must be split into two returns
- If you change your company's year-end, HMRC will adjust your filing obligations accordingly
- The CT600 filing deadline is 12 months after the end of the accounting period (per GOV.UK guidance)
What Happens If You Don't File?
Failure to file a CT600 by the deadline results in automatic late filing penalties. These begin at a flat-rate penalty immediately after the deadline and increase for returns that are significantly overdue. If HMRC believes you have taxable income, they may also raise a tax assessment based on their own estimate of what you owe.
Repeated failure to file can result in larger penalties and, in serious cases, criminal prosecution.
For a full breakdown of what to expect, see our guide to CT600 late filing penalties.
Quick Reference: Do You Need to File?
| Situation | CT600 Required? |
|---|---|
| Trading limited company | Yes |
| Limited company making a loss | Yes |
| Limited company with no taxable income | Yes (if HMRC issued a notice) |
| New company, not yet trading | Yes (until HMRC confirms dormancy) |
| Company confirmed dormant by HMRC | No (unless new notice issued) |
| Sole trader | No (use Self Assessment) |
| Partnership | No (use SA800) |
| Registered charity (meeting conditions) | Usually no |
Summary
Most UK limited companies must file a CT600 company tax return, even if they have no profit or no tax to pay. The obligation arises when HMRC issues a notice to deliver — which happens automatically when a company is incorporated.
Dormant companies can become exempt once HMRC formally confirms dormancy, but must typically file at least one return to establish that status. Sole traders and partnerships are outside the scope of the CT600 entirely.
If you are unsure whether your company needs to file, the safest course is to assume it does — and either file a return or contact HMRC to confirm your position. The cost of missing a deadline is far greater than the effort of filing a nil or dormant return.