How to File a Corporation Tax Return (CT600) Online

A Company Tax Return tells HMRC how much Corporation Tax your company owes. It normally includes the CT600, company accounts and a tax computation. Filing the return, paying the tax and filing accounts at Companies House are separate tasks.

Check whether a return is required

If HMRC sends a notice to deliver a Company Tax Return, you must respond even if the company made a loss or owes no tax. If you believe the notice should be withdrawn because the company is dormant, contact HMRC; do not simply ignore it. See dormant company filing.

A company tax accounting period cannot exceed 12 months. First accounts covering longer than 12 months can therefore require two returns. Confirm your HMRC accounting periods before preparing figures or choosing software.

Filing and payment deadlines

The normal filing deadline is 12 months after the accounting period ends. For companies outside the instalment-payment rules, Corporation Tax is normally payable nine months and one day after that period ends. Larger companies can have earlier instalment deadlines.

For example, for an accounting period ending 31 March 2026, the normal payment deadline is 1 January 2027 and the return deadline is 31 March 2027. Arrange payment early enough to arrive on time. Filing a return does not pay the bill.

See how to pay Corporation Tax for payment methods and references.

Gather the filing information

• The company's 10-digit Corporation Tax UTR, company number and accounting-period dates.
• The Government Gateway access needed for Corporation Tax, or an authorised agent's filing arrangement.
• Approved accounts, including the profit and loss account, balance sheet and relevant notes.
• A reconciliation from accounting profit to taxable profit, including disallowable costs and capital allowances.
• Details of losses, other associated companies, distributions and any items requiring supplementary pages.
• Prior submissions and HMRC notices, so you avoid filing the wrong period or sending a duplicate return.

Your personal Self Assessment UTR is not the company's UTR. Use the UTR guide if you are unsure which reference you have.

Use a current filing route

HMRC's free Company Accounts and Tax Online (CATO) service closed on 31 March 2026. Signing into a business tax account does not provide the old CT600 filing form. Most companies now use compatible commercial software or an accountant. Limited exceptions to online filing are explained in HMRC's guidance.

Before buying software, check that it supports your period end, accounts format and tax circumstances. In particular, check long first periods, losses, directors' loans, group arrangements and supplementary pages. A bookkeeping export alone is not a completed Company Tax Return.

Prepare, review and submit

1. Reconcile the books and approve the statutory accounts. Explain differences between accounting profit and taxable profit in the computation.
2. Prepare the CT600 and any necessary supplementary pages. Accounts and computations normally need iXBRL tagging; a PDF attachment is not an equivalent substitute.
3. Review company identity, period dates, profit adjustments, losses, rates and marginal relief. Check that the accounts and computation agree with the return.
4. Complete the software's validation and declaration steps. Resolve errors before submission.
5. Submit once and retain the HMRC acceptance response and the exact documents filed. If the outcome is uncertain, check status before attempting another submission.
6. Pay the liability using the correct payment reference and retain evidence of payment separately.

An upload or payment confirmation is not proof that HMRC accepted the return. Likewise, HMRC acceptance does not prove that Companies House accounts have been filed.

Using Unfiled

You can prepare and file supported CT600 returns directly with HMRC through Unfiled. Start from bank transactions or ready year-end figures, review the statutory accounts and tax computation, complete the declaration, then choose Submit to HMRC. Unfiled submits on your instruction and shows HMRC's response.

Unfiled checks your company, accounting period and tax circumstances before payment and tells you if your return is outside its supported scope. Keep the HMRC acceptance confirmation; downloading a filing pack does not submit it, and filing does not pay your Corporation Tax bill.

Prepare and file your CT600 with Unfiled and review current pricing. For the Companies House obligation, use our separate accounts filing guide.

Official sources

Reviewed 30 September 2026: Company Tax Returns, return deadlines and filing after the CATO closure.