Written by the TaxTune Editorial Team · Reviewed by our qualified accountants · Updated for 2026
The quick answer The CT600 is the company tax return you file with HMRC each year. It reports your profit, works out the corporation tax due, and is filed alongside your company accounts and tax computations. It must reach HMRC within 12 months of your accounting period end.

What the CT600 contains

  1. Company and period details. Your company number, UTR and the accounting period covered.
  2. Turnover and profit. Your income and the taxable profit after adjustments.
  3. The tax calculation. The corporation tax due, after rates, marginal relief and any allowances.
  4. Reliefs and claims. Capital allowances, losses and any other reliefs your company claims.

What you file with it

The CT600 does not travel alone. You file it with your statutory accounts and the tax computations that show how the profit becomes the tax. Your accounts also go to Companies House, on their own deadline.

Deadlines and penalties

File the CT600 within 12 months of your period end. Miss it and there is an automatic £100 penalty, another £100 after 3 months, and percentage based penalties after 6 and 12 months. Remember the tax itself is due earlier, at 9 months and 1 day.

Filing a CT600 yourself?

The CT600, computations and accounts all have to agree, and the software is built for accountants. TaxTune prepares and files the lot, correctly and on time, so you never face a rejected return.

See our company accounts serviceAsk about your CT600

Let us prepare and file your CT600

We handle your accounts, computations and CT600 together, claim every allowance, and file with HMRC and Companies House on time. Fixed fee.

Get a fixed fee quoteTalk to a real accountant

Keep reading

Frequently asked questions

What is a CT600?

It is the company tax return a limited company files with HMRC each year. It reports profit and works out the corporation tax due, filed alongside the accounts and tax computations.

When is the CT600 due?

Within 12 months of the end of your accounting period. Note that the corporation tax payment itself is due earlier, 9 months and 1 day after the period end.

What do I file with the CT600?

Your statutory accounts and the tax computations that show how the profit becomes the tax. The accounts also go to Companies House on their own deadline.

What is the penalty for a late CT600?

An automatic £100, another £100 after 3 months, then tax based penalties after 6 and 12 months. Persistent lateness increases the fixed penalties.

Can I file a CT600 myself?

You can, but it must agree with your accounts and computations and usually needs commercial software. Many directors use an accountant to avoid rejected or incorrect returns.