CT600 is the Corporation Tax return that UK limited companies must file with HMRC. It reports your company's taxable profits and calculates the tax due.

What is the difference between CT600 and annual accounts?

Annual accounts are filed with Companies House and show your company's financial position. The CT600 is filed with HMRC and calculates your corporation tax liability. Both are required but filed separately.

When is the CT600 deadline?

The CT600 is usually due 12 months after the end of your accounting period. The corporation tax payment deadline is earlier (typically 9 months and 1 day after period end).

What CT600 software do I need?

You typically need software or a service that can submit CT600 online in iXBRL format (often including iXBRL accounts and computations). Very simple cases may use HMRC online services, but most companies use commercial CT600 software or an agent.

When do I have to pay corporation tax?

Corporation tax is typically payable 9 months and 1 day after the end of your accounting period, which is before the CT600 filing deadline.

Do micro-entities need to submit full accounts to HMRC with CT600?

Yes. Even if you file simplified micro-entity accounts to Companies House, you still need full accounts as part of the CT600 submission to HMRC.

Is CT600 filed with Companies House?

No. CT600 is filed with HMRC. Companies House receives annual accounts separately.

What happens if CT600 is filed late?

HMRC can charge penalties for late CT600 filing and interest on late tax payments. The exact penalty depends on how late you are and whether tax is unpaid.

CT600 Corporation Tax Return Guide UK 2024 - HMRC Filing

1. What is a CT600?

The CT600 is the Corporation Tax return that all UK limited companies must file with HMRC. It calculates how much corporation tax your company owes based on its taxable profits.

Corporation Tax Rate 2024/25

The main rate is 25% for profits over 250,000 GBP. Small profits rate is 19% for profits under 50,000 GBP. Marginal relief applies for profits between 50,000 and 250,000 GBP.

2. CT600 vs Annual Accounts

Many company directors confuse these two filings. They are different and both required.

Aspect	Annual Accounts	CT600
Filed with	Companies House	HMRC
Purpose	Show financial position	Calculate tax owed
Deadline	9 months after year end	12 months after year end
Public record?	Yes - visible to anyone	No - confidential
Format	iXBRL (micro-entity can be simpler)	iXBRL with full accounts

Important

While micro-entities can file simplified accounts with Companies House, they must still submit full accounts with their CT600 to HMRC.

3. Filing Deadlines

Requirement	Deadline	Example (31 March year end)
Pay Corporation Tax	9 months + 1 day	1 January
File CT600	12 months	31 March (next year)

Pay Before You File

The payment deadline (9 months + 1 day) is BEFORE the filing deadline (12 months). You need to estimate your tax bill and pay on time to avoid interest charges.

4. What to Include in Your CT600

Full accounts - Balance sheet and profit/loss account
Tax computations - How you calculated taxable profit
Supporting schedules - Capital allowances, losses, etc.

Copy-paste checklist (before you start)

I know my accounting period end date and my deadlines (tax payment vs CT600 filing).
I have full accounts ready for HMRC (P&L + balance sheet), even if I file simplified micro-entity accounts to Companies House.
I have a list of adjustments (capital allowances, disallowable expenses, losses) if applicable.
I have Government Gateway access (or I will use commercial software that files for me).

5. How to File Your CT600 (Software + Submission)

CT600 must be filed online in iXBRL format. In practice, this usually means using CT600 software (or an agent) that can submit iXBRL accounts and computations to HMRC.

HMRC Online Services

Free but requires manual data entry. Best for very simple situations.

Commercial Software

Generates iXBRL automatically and files directly to HMRC.

Quick checklist (before you pick CT600 software)

Can it submit CT600 + iXBRL accounts to HMRC?
Does it handle your complexity (losses, capital allowances, director loans, VAT/PAYE interactions)?
Do you want a DIY tool, or an agent/accountant filing service?

Source: Gov.uk - Company Tax Returns

6. Common Mistakes to Avoid

Confusing filing with payment deadline

Tax payment is due 3 months BEFORE the CT600 filing deadline.

Not including full accounts

Even micro-entities must submit full accounts with CT600.

Forgetting capital allowances

Make sure to claim allowances for equipment and vehicles.

CT600 Corporation Tax Return Guide UK 2024

In This Guide