What happens if you don't declare crypto to HMRC?

Let's start with the thing that's actually on your mind. Tax evasion is technically a criminal offence, but it requires dishonest intent. For an ordinary trader who forgot, misunderstood the rules, or didn't realise a swap counted, that intent simply isn't there, so HMRC treats the matter as civil: back-tax you should have paid, interest on it, and a penalty. Criminal prosecution is reserved for deliberate omission combined with active concealment (false records, lying to HMRC), and it is genuinely rare. So the honest framing isn't “you're in trouble”; it's “there's a bill to sort out, and how you sort it out changes the size of it.”

The single most important fact is this: coming forward yourself, before HMRC contacts you, is far cheaper than being caught. The penalty for the same mistake can be nil if you disclose it unprompted, or a large fraction of the tax if HMRC finds it first. Everything below explains what actually happens, honestly, so you can make the calm, informed choice rather than the frightened one.

What actually happens if you do nothing

HMRC cannot reach back and punish you for gains it has never discovered. That's the reassuring half. The uncomfortable half is that discovery is now close to certain, and on a clock. From 1 January 2026, UK crypto platforms must collect your identity and transaction data and report it to HMRC under the Cryptoasset Reporting Framework (CARF). The first reports cover platforms' 2026 transactions and land on HMRC's desk by 31 May 2027, with later years reported annually after that. From then, HMRC can line those third-party records up directly against what you did or didn't declare.

One thing to be clear about: CARF is forward-looking. It covers transactions from 2026 onwards, so it won't itself report your 2024 or 2025 activity. But that does not make earlier years safe. Undeclared gains from before 2026 can still surface through HMRC's existing data requests to exchanges and its international data-sharing arrangements, which have been feeding nudge letters for years. So “do nothing” usually plays out one of two ways. Either you come forward before that data arrives, on the best possible terms, or you wait, the data arrives, HMRC spots the gap, and you get a nudge letter or a formal enquiry on far worse terms. HMRC already sent around 65,000 crypto nudge letters in 2024/25, roughly double the year before, and that was before CARF even started. The trend only goes one way.

The thing that changed

“They'll never know” used to be a gamble. Now it isn't.

The old bet was that HMRC couldn't see on-chain or exchange activity. From 2026 that bet is off. Under CARF, exchanges hand HMRC your name, tax residency and transaction data automatically, and HMRC cross-checks it against your Self Assessment. A gap between what an exchange reports and what you filed is exactly the kind of red flag that opens an enquiry. This isn't a reason to panic; it's a reason to get ahead of it while coming forward still counts as voluntary.

The penalties, in plain numbers

Penalties sound scary in the abstract, so let's make them concrete. A penalty is a percentage of the tax you should have paid (HMRC calls this the Potential Lost Revenue, or PLR), not a percentage of your whole portfolio. The percentage depends on two things: how the omission is classified (your behaviour), and whether you tell HMRC before or after they come knocking (unprompted versus prompteddisclosure). That second lever is the one within your control, and it's huge.

There are two penalty regimes, and which one applies depends on whether you filed a return. If you filed a Self Assessment return but left crypto gains off it, that is a Schedule 24 inaccuracy(Finance Act 2007). If you didn't file a return for the year at all, or filed without disclosing that you traded crypto, that is a Schedule 41 failure to notify(Finance Act 2008). Both lead to broadly the same penalty bands, but the classification can affect HMRC's starting position and, importantly, how far back HMRC can go (more on that below).

Behaviour is graded on a spectrum, and the bands work like this:

Careless (an honest mistake, or not taking reasonable care): standard penalty is up to 30% of the tax due. Disclose it unprompted and it can be reduced to nil. Wait until HMRC prompts you and the minimum is 15%.
Deliberatebut not concealed (you knew and chose not to report, but didn't actively hide it): standard penalty is up to 70%. Unprompted, the minimum is 20%; prompted, 35%.
Deliberate and concealed (you knew, chose not to report, and took steps to hide it, such as false records): standard penalty is up to 100%. Unprompted, the minimum is 30%; prompted, 50%.

For most ordinary people who simply didn't realise a swap or a sale was taxable, the honest classification is careless, not deliberate. And careless plus unprompted can mean no penalty at all on top of the tax and interest. That is the single strongest reason to act now rather than wait. Which band applies is genuinely fact-dependent, and HMRC and you may not agree at first; a good, full disclosure is how you make the case for the gentler band.

Worked example

The same £10,000 undeclared gain, two very different outcomes

Say you had a £10,000 undeclared gain a couple of years back. After your allowance and at, say, a 20% effective rate, the tax owed (the PLR) works out at roughly £2,000. If HMRC accepts this was careless:

You come forward unprompted, and HMRC accepts it was careless: the penalty can be reduced to £0. You pay the £2,000 tax plus interest, and that's it. (If HMRC instead treats it as deliberate, the minimum is 20% even when unprompted, which is why a full, honest disclosure that argues for the careless band matters so much.)
HMRC catches it first (prompted): minimum 15% penalty, so at least £300 on top of the £2,000 tax and interest.

The gap looks modest here, but scale the gain up, or land in the deliberate band, and the difference between coming forward and being caught can run into thousands. The tax and interest are owed either way; the penalty is the part your timing controls.

A note on non-UK exchanges

There's a wrinkle worth knowing honestly. Where undeclared gains sit on non-UK exchanges (the international arms of the big platforms), HMRC can apply an offshore penalty uplift, because it treats offshore non-compliance as higher-risk. Two things keep this in proportion. First, the uplift bites on deliberate offshore non-compliance, not on an ordinary careless omission, so for someone who traded without realising and is now coming forward voluntarily, it is unlikely to apply at all. Second, unprompted disclosure of a careless omission keeps the penalty in the nil-to-30% band whether the exchange was UK or offshore. The uplift is a real consideration only for deliberate, concealed non-disclosure, where in the most serious cases it can raise the maximum to around 200%of the tax due (versus 100% for the equivalent onshore matter). Even then, unprompted disclosure provides substantial mitigation. So the takeaway isn't “offshore is scarier”; it's that using a non-UK exchange doesn't make you safer, and coming forward is still the move. Whether an uplift applies to your case is fact-dependent.

Interest, which applies whatever else happens

Separate from any penalty, HMRC charges late-payment interest on tax paid after its due date (31 January following the tax year the gain fell in). The rate is set at the Bank of England base rate plus 4%, and it accrues daily. Because it moves with the base rate, check GOV.UK for the current HMRC interest rate. Interest isn't a punishment, it's compensation for paying late, so it applies even where the penalty is nil, and voluntary disclosure doesn't waive it. The one thing that shrinks the interest bill is time: the sooner you pay, the less it adds up. Another quiet argument for acting now rather than next year.

How far back HMRC can go

This is the other big worry, so here it is straight. HMRC's normal window to assess is 4 years from the end of the tax year. But it extends:

4 years as standard.
6 years where the tax was lost through carelessness, including an inaccuracy on a return you did file.
20 years where the loss was deliberate, or where there was a failure to notify chargeability at all (that is, you never told HMRC and never filed for the year).

The distinction matters, so don't let the 20-year figure panic you. That outer limit applies to complete non-disclosure (failure to notify) or genuinely deliberate behaviour. If you did file a Self Assessment return, even late or incomplete, and simply got the crypto figures wrong through carelessness, that falls under the 6-yearwindow, not 20. The reason 20 years is often quoted for undeclared crypto is that many people never filed for the years in question at all, which puts them in the failure-to-notify category. It's the ceiling for the most serious cases, not the default for everyone. What it really tells you is not to gamble on the years “ageing out”: with CARF data arriving from 2027 and these extended windows available to HMRC, waiting for the clock to save you is not a plan you can rely on.

How HMRC actually finds out

It helps to see the machinery, because “maybe they won't notice” tends to carry a lot of weight in most people's heads. Here's how the gaps get spotted.

CARF, from 2026.UK crypto platforms report your identity, residency and transactions to HMRC automatically, first reports due 31 May 2027. This is third-party data you don't control. See which exchanges report to HMRC and the fuller CARF explainer.
Nudge letters. HMRC sent roughly 65,000 in 2024/25, built on earlier exchange data and pattern-matching. A letter means HMRC already thinks you traded and has no record of tax paid. If one has already arrived, read what a nudge letter means and what to do.
Direct data requests. HMRC can and does ask UK-regulated exchanges for user data directly, which is how it reached earlier years before CARF existed.
International data-sharing. From 2027, CARF data flows between tax authorities, so activity on a non-UK exchange can still find its way back to HMRC.
Your own money trail.Fiat moving in and out of a UK bank or a regulated exchange leaves a record that may not match what you've filed.

The advantage of coming forward first

Here is the genuinely reassuring part, and the point of this whole guide. HMRC runs a voluntary disclosure route specifically for this: the Cryptoassets Disclosure Service (part of HMRC's wider Digital Disclosure Service). Using it before HMRC contacts you is what locks in the unprompted penalty bands, which is where the real saving lives. Our step-by-step guide to making a voluntary disclosure walks the whole process. In outline:

Notify. Tell HMRC you intend to disclose. Making that first notification, before any nudge letter or enquiry, is what establishes you as coming forward voluntarily. This is the moment that matters: once HMRC contacts you first, your status flips to prompted and the minimum penalties jump.
Work out the real number.Calculate the gains and losses for every year involved, HMRC's way: the Section 104 pool with the same-day and 30-day rules, every disposal priced in pounds at the moment it happened.
Disclose in full. Submit the detail: exchanges used, dates, amounts, the lot. Full, honest transparency is exactly what earns the maximum penalty reduction; a partial or vague disclosure works against you.
Settle. HMRC works out the tax, interest and penalty, and issues a demand. Payment is usually due within 30 days, though time-to-pay arrangements can be negotiated.

The payoff is the difference between the unprompted and prompted bands set out above: careless and unprompted can be a nil penalty; caught first, it starts at 15% and climbs. The practical deadline to preserve unprompted status is before the first CARF reports land in May 2027. After that, HMRC has the exchange data in hand, and a disclosure made once they can already see the gap is treated as prompted.

Could this be criminal? Almost always no.

This is the worry that tends to loom largest, so here it is in proportion. Civil penalties are the overwhelming norm. Criminal prosecution for tax evasion is rare and reserved for the most serious, deliberately dishonest cases: false documents, destroyed records, lying to HMRC, systematic concealment across years, or large sums combined with active dishonesty. (For context, the maximum sentence for the most serious income-tax fraud rose to 14 years under the Finance Act 2024, but that ceiling is for exactly those extreme cases.) Forgetting to report gains, or misunderstanding that a swap was taxable, is not that.

For a typical trader who used an exchange, didn't realise the gains were reportable, and then comes forward voluntarily, the realistic criminal risk is very low. HMRC settles those cases civilly. The behaviour that actually raises criminal risk is the opposite of disclosure: ignoring a nudge letter, then filing false returns claiming no gains. A voluntary, unprompted disclosure is a strong signal of good faith and makes criminal prosecution exceptionally unlikely in ordinary cases, though HMRC does retain discretion. In practice, the civil disclosure route is nearly universal for someone who comes forward honestly. Criminal cases tend to follow a failure to engage, not a decision to come clean.

What to do now

If your head is spinning, here's the calm version. The worst move is to keep ignoring it and hope the CARF data misses you; it won't. The best move is almost always the same, whether you end up owing a lot or nothing:

Work out the real number first. Before you assume the worst, calculate it. Many people owe far less than they fear once losses, the annual allowance, and untaxed transfers between your own wallets are taken into account. Some owe nothing at all.
If you owe for earlier years, use HMRC's Cryptoassets Disclosure Service to come forward unprompted, while that status is still yours to claim.
For the current year, report it properly on the SA108 pages of your Self Assessment.
Keep your workings, whatever the outcome, so you can show HMRC exactly how each figure was reached.

The tedious part is the arithmetic: matching every disposal under the pooling rules, often across years and more than one exchange, priced in pounds at the moment of each trade. That's the job CryptoCGTis built for. You upload your transaction history (a CSV from any exchange), and it works out the gains under HMRC's Section 104 rules, gives you the SA108 figures, and hands you a dated, locked copy of the computation, so you have the exact number to disclose and a record of how it was reached. It reproduces HMRC's own worked examples to the penny. One flat fee, no subscription. Always check the figures against your own records before you file or disclose.

Not even sure crypto tax applies to your situation? Start with how UK crypto tax works. This is general information, not personal tax advice. If the amounts are material, if several years are involved, or if your history is large or messy, it is well worth speaking to a qualified adviser before you disclose; a good adviser can also help argue the gentler behaviour band on your behalf.

Sources

See your own number — free, no account

Drop your exchange CSV and read your full Capital Gains Tax figure on screen, with the Section 104 working shown. You only pay if you download the report.

Start free →

This guide is information, not tax advice.Figures and thresholds are for the tax year shown (England, Wales & Northern Ireland; Scottish income tax bands differ). Rates and rules can change, and your own position may differ — check your circumstances and speak to an accountant before you file. CryptoCGT is an information tool, not a regulated tax adviser.