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If you have taxable crypto activity in the UK, meeting HMRC’s deadlines is as important as calculating the tax correctly. Late returns, incorrect reporting, and late payments can trigger automatic penalties and interest — sometimes large amounts — even when the underlying tax bill is modest.
This guide explains the key UK tax deadlines for the 2025–2026 tax year, how penalties are calculated, how interest is charged on unpaid tax, and what happens if you file or pay late.
Key Deadlines for Crypto Tax in the UK
The UK tax year runs from 6 April to 5 April. All crypto disposals and income within that period are reported together.
Registering for Self Assessment (Notification)
If you need to file a Self Assessment return and have not been told to file by HMRC, you must notify HMRC by 5 October 2026 for the 2025–2026 tax year.
Safe Assumption: Don’t rely on HMRC leniency. Missing the 5 October deadline can attract a failure-to-notify penalty, even if you file later.
Filing Deadlines (2025–2026 Tax Year)
| Filing Method | Deadline |
|---|---|
| Paper Return | 31 October 2026 |
| Online Return | 31 January 2027 (Midnight) |
Payments on Account — An extra tax bill
If your total tax bill exceeds £1,000 for 2025–2026, you are normally required to make Payments on Account for the next year. This is basically an advance instalment payment.
Example: The "Double Bill" Effect
If you owe £2,000 for 2025-26:
- Tax due for 2025-26£2,000
- 1st Payment on Account (for 26-27)+ £1,000
- Total Due on 31 Jan 2027£3,000
This is often why crypto users face unexpectedly high bills in January.
Reducing Your Payments on Account
If you know your tax bill will be lower next year (e.g. your crypto profits have dropped), you can ask HMRC to reduce your payments on account.
Online Method (Preferred)
Sign in to your HMRC online account, select "View your latest Self Assessment return", and choose "Reduce payments on account".
Postal Method
Download and complete Form SA303 and post it to HMRC.
Warning: If you reduce payments too much and underpay, HMRC will charge interest on the shortfall. Be realistic with your estimates.
Mandatory Crypto Reporting (SA108)
Established for 2025-2026
HMRC now requires crypto disposals to be reported through the dedicated cryptoasset section on the Capital Gains Tax summary (SA108).
You must separate crypto from "other assets". Failing to complete this section correctly can trigger accuracy-related penalties.
Late Filing Penalties
Filing a return late triggers automatic penalties, regardless of whether tax is due.
- £100 Automatic
- Applied immediately after midnight on the deadline, even if you owe nothing.
- Daily Penalties
- After 3 months: £10 per day, up to £900 maximum.
- 6 & 12 Month Penalties
- Additional £300 or 5% of tax due (whichever is higher) at each stage. Higher if deliberate.
Late Payment Penalties & Interest
Penalty Stages
| Time Late | Penalty |
|---|---|
| 30 days | 5% of unpaid tax |
| 6 months | Additional 5% |
| 12 months | Additional 5% |
Interest on Late Tax (Variable Rate)
HMRC charges interest from the day after the deadline until tax is paid.
HMRC late-payment interest is variable
It is generally set at Bank of England Base Rate + 4 percentage points and can change over time. Check HMRC’s current rate when calculating exposure.
Reasonable Excuse
HMRC may cancel penalties for "reasonable excuses" like serious illness, bereavement, or system failures. Being unaware of the rules or "too busy" is not accepted.
Common Misunderstandings
- “If I owe no tax, there’s no penalty for late filing”
- False. The £100 automatic penalty applies even if no tax is due.
- “Interest only applies after penalties”
- False. Interest starts accruing immediately from the day after the due date.
- “Crypto has special deadlines”
- False. Crypto reporting follows strict standard Self Assessment deadlines.
Summary
- For 2025–2026, register by 5 October 2026 and file online by 31 January 2027.
- Pay tax and any first Payment on Account by 31 January 2027.
- Late filing triggers £100 automatic penalty + daily charges.
- Late payment triggers 5% penalties plus variable HMRC interest.
- Mandatory crypto section on SA108 must be used.
- Errors are easier for HMRC to spot with new reporting frameworks (CARF).
Worried about taxes and deadlines?
BlockBooks combines specialist crypto tax experience with careful review to calculate your position accurately and provide the figures you need for your SA108 form.