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

HMRC can charge the unpaid tax plus interest and penalties — and penalties are much higher when non-disclosure is deliberate. UK exchanges already share customer data with HMRC, and CARF reporting from January 2026 widens that net, so undeclared gains increasingly trigger nudge letters or compliance checks. If you are behind, a voluntary disclosure before HMRC contacts you usually means lower penalties.