From 26 March 2026, the inclusion of a Trailer Reference Number (TRN) on a GMR will be mandatory if the movement has a trailer.
When creating a GMR, you will be asked if the vehicle has a trailer. If yes, you will be required to enter the TRN at a later point in the GMR creation process. You will not be able to complete a GMR without the TRN.
HMRC
