How to File a Corporation Tax Return (CT600) Online

Every active UK limited company must file a Corporation Tax return (CT600) with HMRC for each accounting period — even if it made no profit. This guide walks you through filing online, end to end.

When to file

• Filing deadline: 12 months after the end of your accounting period.
• Payment deadline: 9 months and 1 day after the end of your accounting period (earlier than the filing deadline).
• Miss the filing deadline and HMRC charges automatic penalties starting at £100.

What you need before you start

1. Your Unique Taxpayer Reference (UTR) — a 10-digit number issued by HMRC.
2. HMRC Government Gateway user ID and password with Corporation Tax enrolled.
3. Your statutory accounts for the period (profit & loss, balance sheet, notes).
4. A tax computation showing how taxable profit is calculated (add-backs, capital allowances, etc.).
5. Your company registration number and registered office address.

Step 1 — Prepare your accounts in iXBRL

CT600 returns must be filed with accounts and computations tagged in iXBRL (a machine-readable format). Most small-company filers do this in one of three ways:

• HMRC's free Company Accounts and Tax Online (CATO) service — for the simplest companies.
• Commercial filing software (Xero, FreeAgent, TaxFiler, etc.).
• An accountant who handles the iXBRL tagging on your behalf.

SoloKit currently surfaces the figures and supporting schedules you need; the CT600 itself is filed manually via HMRC online or your chosen filing tool.

Step 2 — Sign in to HMRC online

Go to gov.uk → Sign in to HMRC → Corporation Tax. You'll see your accounting periods listed. Pick the one you're filing for.

Step 3 — Complete the CT600 form

Enter:

• Trading and other income.
• Allowable expenses and add-backs (entertainment, depreciation, etc.).
• Capital allowances (AIA, full expensing, writing-down allowances).
• Losses brought forward or surrendered.
• Tax due at the relevant rate (19% small profits rate, 25% main rate, marginal relief in between).

Step 4 — Attach accounts and computations

Upload your iXBRL-tagged accounts and the computation. HMRC validates the file before submission.

Step 5 — Submit and pay

After submission you'll get a confirmation receipt. Pay the tax due 9 months and 1 day after the period end via Faster Payments, BACS, CHAPS or direct debit. Use your 17-character Corporation Tax payment reference (UTR + period suffix).

Common mistakes to avoid

• Filing the previous period instead of the current one.
• Forgetting to add back disallowable expenses.
• Missing the payment deadline (it's earlier than the filing deadline).
• Not telling HMRC the company is dormant if it traded for only part of the period.

Need to file a nil or dormant return?

If the company didn't trade, see our Dormant Company Filing guide — you may not need a CT600 at all.

Summary

Filing CT600 online is straightforward once you have the UTR, Gateway access and iXBRL-tagged accounts. Pay first (9m+1d), file second (12m). For the rates and reliefs, see Corporation Tax for Small Companies.