Seventeen thousand six hundred people declared a cryptoasset capital gain to HMRC for the 2024/25 tax year, on disposal proceeds of £13.8bn. HMRC published that for the first time on 27 August 2026. Set against the number of people who have traded crypto in the UK it is small, and the gap is the reason this article exists.
If you are in that gap, the useful first question is not how much you owe. It is how many years HMRC can reach back over, because that answer moves the bill more than any arithmetic you do afterwards.
Four years, six, or twenty
HMRC’s disclosure guidance sets three windows by behaviour: four years for reasonable care, six for carelessness, twenty for a deliberate underpayment. Most people read that, decide they were careful, and settle on four.
That is usually the wrong answer, for a reason unconnected to how careful anyone was. The four and six year limits both assume you registered for Self Assessment by the deadline. If you never registered at all, this is a failure to notify, and CH53600 is blunt: “the time limit for failure to notify is 20 years whether or not the failure was deliberate”. Someone who bought in 2018, sold in 2021 and never told anyone is not a careless filer. They are a non-filer, and the window is twenty years.
A separate twelve-year limit covers offshore matters. Whether an account with a non-UK exchange makes a matter offshore is a live judgement, and it is the judgement that turns a four-year problem into a twelve-year one. There is no clean answer to pretend about.
The route, and what it does not cover
The service is the one GOV.UK titles “Tell HMRC about unpaid tax on cryptoassets”, which HMRC elsewhere calls its Cryptoasset Disclosure Service. It takes capital gains and income tax on tokens, and you work out the tax, interest and penalty yourself.
It does not take the current tax year or the one before it; those go on a return. So 2025/26 gains are not a disclosure at all: register for Self Assessment by 5 October 2026 and file by 31 January 2027.
What the delay actually costs
Say £10,000 of capital gains tax was due for 2022/23, payable on 31 January 2024 and never paid. Interest runs daily from 1 February 2024 at whatever rate is in force, and the rate moved seven times over that period, ending at 7.75 per cent from 9 January 2026. Across 957 days that is £2,036.99, or 20.4 per cent of the tax.
Then the penalty. A non-deliberate failure to notify disclosed without prompting, more than twelve months after the tax was due, carries 10 to 30 per cent. HMRC restricts the reduction where disclosure has taken three years or more, capping the benefit ten points above the floor, so the best outcome is 20 per cent, not 10. Wait for a letter and the range starts at 20, and the restriction pins you at 30.
| Tax | £10,000.00 |
| Interest, 957 days, eight rate periods | £2,036.99 |
| Penalty, unprompted | £2,000.00 (20%) |
| Penalty, after a nudge letter | £3,000.00 (30%) |
| Total, telling them first | £14,036.99 |
| Total, waiting to be asked | £15,036.99 |
| Late payment interest rate | 7.75% from 9 January 2026 |
| Earliest year open, non-deliberate | 2009/10 |
A thousand pounds on a ten thousand pound liability is the price of the letter arriving first, and CARF makes the letter likelier: UK providers began collecting customer data on 1 January 2026 and HMRC receives the first reports by 31 May 2027.
The case for not panicking
The bear case is above; the bull case is real too. Reasonable care attracts no penalty at all, the disclosure is self-serve rather than an investigation, and HMRC accepts estimates where records are gone, provided you say on the return that the figures are estimated or provisional and keep your workings. If you cannot pay, Time to Pay can be set up online for bills up to £30,000 — a threshold HMRC stated in December 2025 and has not restated. Interest runs either way.
Where this stops being a website’s job: anything that might be characterised as deliberate, where the Contractual Disclosure Facility is the only route carrying protection from criminal investigation; any non-UK exchange or custodian; and rebuilding pooled costs across dead exchanges. HMRC’s own interest and penalty calculators stop at 5 April 2024, which tells you how much of this is meant to be done by hand.
Sources, all retrieved 14 September 2026: GOV.UK “Tell HMRC about unpaid tax on cryptoassets”, “How to make a voluntary disclosure to HMRC”, Self Assessment deadlines and penalties, “If you cannot pay your tax bill on time”, and HMRC interest rates for late and early payments; Compliance Handbook CH53100, CH53400, CH53600 and CH53700; factsheets CC/FS11 and CC/FS7A; HMRC news release “240 crypto millionaires revealed in new government data”, 27 August 2026. The interest figure was computed here, daily, over eight rate periods running from 22 August 2023; the £10,000 liability is illustrative. CH53600 carves 2008/09 and earlier out of the 20-year rule unless the loss was negligent, so 2006/07 is reachable only on deliberate behaviour. Contested or unverified: HMRC does not publish nudge letter volumes, so the widely quoted 65,000 and 81,000 figures are one accountancy firm’s analysis and are not used in the arithmetic above. The £30,000 Time to Pay threshold is verified only to 9 December 2025. Whether a non-UK exchange makes a matter “offshore”, and whether Time to Pay suspends the late payment penalties during a disclosure, are unresolved on the published guidance. Not a recommendation to buy, sell or hold anything, and not tax advice.