For years, crypto investors have operated in a grey area where HMRC could ask about your transactions but rarely had the data to find them on its own. That is about to end. The OECD's Crypto-Asset Reporting Framework, known as CARF, starts collecting UK data this year, and the first international exchanges of that data happen in 2027. If you have unreported crypto gains from previous years, the window to disclose them voluntarily is closing.
This guide explains what CARF is, why it changes the picture, how UK crypto tax actually works, and what to do if you have unreported gains from previous years.
What CARF is and when it kicks in
CARF is an OECD framework for the automatic exchange of information about crypto transactions, similar in principle to the Common Reporting Standard that already moves bank account data between tax authorities. The UK signed up in 2023 and is implementing it through legislation that takes effect in stages.
The 2026 calendar year is the first reporting period for UK crypto service providers. Exchanges, custodial wallet providers, brokers and certain DeFi platforms operating in the UK have to collect information about their users and their transactions throughout 2026 and submit their first reports to HMRC between January and May 2027. The first international exchanges of that data with other tax authorities follow from 2027 onwards.
Once it is in, HMRC will have a structured record of reportable transactions you made on in-scope platforms, including dates, values, and your identity as the account holder. There will not be a "did they have the data" question any more.
Why this changes the picture
Until now, HMRC could only get crypto data through specific information notices to UK exchanges, which take time, lawyers, and a degree of suspicion. There were also enforcement campaigns where HMRC asked some exchanges to hand over user lists, but these were piecemeal. From 2027, the data flows automatically and continuously, and HMRC will be in a much stronger position to cross-check tax returns against CARF data.
Even non-UK exchanges in CARF jurisdictions will report. The framework includes most major economies, so using a foreign exchange does not put you out of reach. The window for "I forgot to mention it" is effectively closing.
How crypto is taxed in the UK
HMRC treats cryptoassets as property for Capital Gains Tax purposes. You owe tax when you dispose of crypto, not when you buy it or hold it. The gain is calculated as the value of what you received minus the cost of what you gave up.
If you actively trade crypto as a business (rare in practice, and HMRC sets a high bar for this), the profits would be taxed as trading income instead of capital gains. For almost everyone, however, crypto gains are CGT.
What counts as a disposal
A disposal is anything that results in you giving up beneficial ownership of a cryptoasset. The obvious examples are selling crypto for fiat currency and swapping one cryptoasset for another. The less obvious examples are spending crypto on goods or services (treated as a disposal of the crypto at its value at the moment of the transaction) and gifting crypto to anyone other than your spouse or civil partner.
Some DeFi activity may also count as a disposal, but the treatment depends on the facts of the specific protocol. HMRC's published guidance turns on whether beneficial ownership of the cryptoasset is considered to pass when you interact with the protocol. Where it does, the transaction is a disposal; where it does not, it is not. Lending and providing liquidity are common areas of doubt and the analysis is fact-specific.
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Try our HMRC Red Flag Checker free, here on this site →What does not count as a disposal
Moving crypto between your own wallets is not a disposal, because you have not given up beneficial ownership. Holding crypto, no matter how much it goes up in value, is not a disposal. Receiving crypto via airdrop is not a disposal in itself, although it may trigger an income tax charge at the time of receipt depending on the circumstances.
How to calculate your gain
Calculating the gain on a single crypto disposal is harder than it sounds because UK pooling rules apply. All your purchases of the same cryptoasset are combined into a "Section 104 pool" with an average cost per unit. When you sell, the cost of the units sold is taken from the pool.
Two anti-avoidance rules sit on top of the pooling rules. The same-day rule says that if you buy and sell the same cryptoasset on the same day, those transactions are matched first. The bed-and-breakfast rule says that if you sell and re-buy the same cryptoasset within 30 days, those are also matched separately from the main pool. The remainder of any sale is then drawn from the pool.
In practice, most crypto investors use software like Koinly, CoinTracker or Recap to handle the pooling calculations automatically. Doing it by hand for any meaningful number of transactions is impractical.
The CGT exemption and rates
The annual CGT exemption is £3,000 for 2026/27, the same level as 2024/25 and 2025/26 and well below the £12,300 it was a few years ago. Gains above the exemption are taxed at 10% if you are a basic-rate taxpayer and 20% if you are a higher or additional-rate taxpayer. These crypto rates are the standard non-residential rates and have not changed in the recent CGT shake-up that affected residential property.
The exemption cuts have pushed many crypto investors into a tax-paying position who would have been below the threshold a few years ago. A £5,000 gain that would have been entirely tax-free in 2022/23 produces £200 to £400 of CGT today.
Mining, staking, and DeFi
Mining and staking rewards are usually treated as miscellaneous income at the time you receive them, valued in pounds at that moment. They then become part of your pool at that value, so any subsequent disposal is a separate CGT calculation on top.
Some DeFi lending or liquidity transactions may amount to a disposal, depending on whether beneficial ownership of the cryptoasset is considered to pass. The analysis is highly fact-specific and depends on how the particular protocol is structured. There is no single rule that covers every case, which is why specific advice is worth the cost for anything beyond casual activity.
HMRC's Cryptoassets Manual covers these areas in more detail, but the rules are still evolving and HMRC's view on specific protocols can change. If you are doing significant DeFi activity, professional advice is worth the cost.
What to do if you have unreported gains
First, calculate the gains for each year you have a problem. Crypto tax software can usually back-calculate from your transaction history if you connect your wallets and exchanges. Where records are incomplete, it is still worth assembling the best evidence you can and taking advice on how to present it in the disclosure.
Second, decide how to disclose. If the omission is recent (within the last 12 months from the original Self Assessment deadline), you can amend the relevant Self Assessment return online. For older years, the right route is the Digital Disclosure Service, which is HMRC's standard channel for unprompted voluntary disclosures.
Voluntary disclosure penalties are typically lower than penalties imposed after HMRC has opened an enquiry, but the actual rates vary significantly depending on the underlying behaviour, the timing, the quality of the disclosure, and whether the matter involves offshore assets. Offshore failure-to-correct cases attract a standard penalty of 200% of the lost tax, reducible depending on the disclosure quality. The general principle is that coming forward voluntarily before HMRC contacts you is almost always cheaper than waiting, but the maths in any individual case depends on the facts.
Why coming forward now is cheaper than waiting
The reason this matters in 2026 specifically is that once CARF reporting is in full flow, it may become much harder to argue that a later disclosure was truly voluntary in any practical sense. HMRC will already have a meaningful body of transaction data when you approach them. The credibility of an "I am volunteering this" framing is best preserved by acting before the data lands.
The Digital Disclosure Service is open today. Voluntary disclosure during the pre-CARF window is the cheapest version of fixing past mistakes you are likely to get.
Next Step
If you have made any crypto disposals in the last few years and have not reported them, calculate your gains now using crypto tax software, then file amended returns where possible or use HMRC's Digital Disclosure Service for older years. For a personalised view of your crypto compliance position alongside any other HMRC red flag areas, use the HMRC Red Flag Checker.
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