From 18 May 2026, HMRC will launch the new online registration system for agent services accounts, which will be mandatory for tax advisory firms, including offshore advisers, with risk of penalties and stop notices for non-compliance.

The new tax adviser register will completely replace the current method of signing up to use HMRC services. But 18 May is a not a hard deadline as there is a staggered timetable so advisers who already have a self assessment or corporation tax account, will not have to register until 18 August, and will have a three-month window before the final registration cut-off date of 18 November.