How long can I carry forward a capital loss?

Indefinitely, once it has been claimed. The claim itself must be made within four years of the end of the tax year in which you made the loss.

Can I claim a capital loss without filing a tax return?

Yes. If you are not in Self Assessment, you can write to HMRC with the details of the disposal and the amount of the loss.

Do brought-forward losses waste my annual exempt amount?

No. They are only used to reduce your gains to £3,000, so the exempt amount is preserved and the rest of the loss carries on.

Can I carry a capital loss back to an earlier year?

Generally no. Capital losses are set against gains of the same year and then carried forward; carry-back is only allowed in the tax year of death.

United Kingdom › Guides

IBKR Capital Losses: Claim Within 4 Years or Lose Them

Updated: 21/10/2026

How to claim capital losses from Interactive Brokers before the 4-year deadline, how HMRC sets them against gains and how to time sales to avoid wasting them.

A capital loss on your Interactive Brokers account only reduces your tax if you report it to HMRC within four years of the end of the tax year in which you made it. A 2025/26 loss must be claimed by 5 April 2030. Once claimed, it can be carried forward indefinitely and set against future gains.

The four-year deadline by tax year

Unlike gains, losses are not automatic. HMRC only lets you use a loss you have formally claimed, and the clock starts at the end of the tax year of the sale:

losses from 2022/23 must be claimed by 5 April 2027,
losses from 2023/24 by 5 April 2028,
losses from 2024/25 by 5 April 2029,
losses from 2025/26 by 5 April 2030.

Losses from 2021/22 and earlier are now out of time. If you sold shares at a loss in 2022/23 and never told HMRC, this is the last tax year in which you can do it.

How to claim a loss

Work out the loss using HMRC's matching rules (same day, next 30 days, then the section 104 pool) in sterling, not IBKR's realised loss figure.
If you file a Self Assessment return, enter the loss on SA108 for the tax year of the sale, in the losses in the year box under listed shares and securities.
If you do not file a return, write to HMRC with the tax year, the shares sold, the dates and the amount of the loss.
Keep your IBKR statements and your calculation, because HMRC can ask how you arrived at the figure.
In later years, enter the brought-forward losses you use against that year's gains on SA108, and keep a running total of what is left.

How HMRC sets losses against gains

The two kinds of loss work differently, and the difference matters:

Losses made in the same tax year are set against that year's gains first, in full, even if that pushes your gains below the £3,000 annual exempt amount and wastes part of it.
Losses brought forward from earlier years are only used to bring your gains down to £3,000. The rest stays available for later years.

For 2024/25, when different rates applied before and after 30 October 2024, you can set losses and the annual exempt amount against the gains taxed at the higher rate first, which gives the lowest bill.

Worked example: brought-forward losses

In 2025/26 you have gains of £10,000 and losses of £2,000 on IBKR, plus £8,000 of losses brought forward. Net gains for the year are £8,000. You use £5,000 of the brought-forward losses to bring that down to £3,000, which the annual exempt amount covers. Tax: £0, and you still have £3,000 of losses for future years.

Worked example: timing a loss-making sale

You are a higher-rate taxpayer with £4,000 of gains so far in 2025/26 and a holding showing a £3,500 loss.

Sell before 5 April 2026: gains £4,000 minus £3,500 = £500, covered by the exempt amount. Tax £0, but £2,500 of the exempt amount is wasted and the whole loss is used up.
Sell after 5 April 2026: you pay tax on £1,000 (£4,000 minus £3,000) at 24%, which is £240, and you carry the £3,500 loss into 2026/27 and beyond.

Which is better depends on what gains you expect next year. If you expect large ones, the carried-forward loss may save more than £240 later.

Common mistakes

Not claiming because no tax was due that year. An unclaimed loss is lost after four years.
Selling for a loss and buying back within 30 days, so the loss is matched with the new shares and disappears.
Working in dollars. A share that is flat in US dollars can show a sterling loss or gain once each trade is converted on its own date.
Forgetting worthless or delisted shares. A negligible value claim can treat them as sold for nothing, creating a loss you can use.
Expecting losses to reduce gains on non-reporting offshore funds. Those gains are taxed as income and capital losses cannot be set against them.

A loss you have already suffered on the market should at least save you tax. Upload your IBKR statement to Brokers.tax and see every loss matched under the same-day, 30-day and section 104 rules, so the losses you claim are ones HMRC accepts, before another deadline slips past.