How to enter brought-forward capital losses in your Corporation Tax return

Last updated: 6 September 2026

Short version. Capital losses brought forward are an unused balance, not an automatic claim for the whole amount. They reduce chargeable gains subject to the applicable rules.

1. Start with the separate figures

Enter gross chargeable gains before capital loss relief, allowable capital losses arising in this period, and unused capital losses brought forward from earlier periods. The opening pool is not necessarily the amount you may deduct. Do not enter the same current-period loss again in the brought-forward field.

The CT600 reports gross gains in box 210, capital losses used in box 215 and net gains in box 220. Box 825 reports losses arising in this period, not the unused closing pool. Your computation shows the carry-forward.

2. Automatic or customer-calculated relief

In the updated filing workflow, ordinary supported cases use current-period losses first and then available brought-forward losses against remaining gains. Where the carried-forward restriction may matter, open “Use your own calculated capital loss relief”. Enter your allowable brought-forward deduction and the company deductions allowance and allocation to gains. We use your calculation rather than replace it with the opening pool. This does not override other relief restrictions or tax rates.

Blank relief selects the supported automatic route. An explicit 0 means no brought-forward relief. If you use the customer-calculated route, enter the allowance details even when an amount is 0. The relief cannot exceed either your available brought-forward losses or the gains remaining after current losses.

If no company allowance is stated, the automatic calculation assumes the maximum allowance for the period. This is a software assumption, not verification of your available allowance. You can use your own calculated capital loss relief even below £5 million if your available allowance is lower.

The manual capital figure does not cover every mixed-loss calculation. If another brought-forward loss or deficit is used and the shared allowance restriction applies, this service may still block the return. It does not yet collect a separate restricted deduction for every other loss stream. A capital claim cannot replace those missing figures. We do not ask you to prove your calculation.

This workflow is being updated. If your version does not show these controls, do not put a restricted deduction in the opening-balance field as a workaround.

3. Two returns for a long period

Assign disposal gains and current losses to the appropriate return in the existing splitter. Enter a separate allowable brought-forward deduction and allowance details for each return when using your calculation. The unused company loss pool passes from the first return to the second. Do not claim the opening balance twice.

For example, with £12,000 opening losses and no current losses, £7,000 used in the first return leaves £5,000. Using £3,000 in the second leaves £2,000. This assumes sufficient allowance and gains in each period. The second return cannot use more than its £5,000 opening pool.

4. Review before filing

If you edit accounting dates, review your entered relief and allowance allocations again. We preserve your figures and recheck their bounds, but do not recalculate a customer-supplied claim for different dates.

Compare your calculation with the generated computation and the CT600. Confirm gross gains minus all capital losses used equals net gains, and opening losses plus current losses minus losses used equals the closing pool. A zero customer claim leaves the brought-forward pool unused. Keep the calculation behind your declaration.

For the £10 million gains example with £5 million allocated allowance, enter £10 million in the opening pool and £7.5 million as your calculated deduction, not £7.5 million as the opening pool. See the capital-loss guide for the assumptions and reduced-allowance example.

Official references

General guidance, reviewed 6 September 2026. Check the rules for your circumstances. Official links do not automatically update our software or this article.