Owners of equines in the UK must have their equines fitted with a microchip.
Previously, owners of equines born before 30th June 2009 did not have to have a microchip fitted, but the law changed in 2018-19 to make it compulsory for all horses to be microchipped, regardless of age.
Equine Register recommends that all owners get their equines microchipped as soon as possible, and don't leave it to the deadlines.
Remember to update your PIO with the microchip number once implanted (you can do this with the Digital Stable).
The deadlines for older horses to be microchipped ("retrospective microchipping") are as follows (there is no deadline for NI):
England: 1st October 2020
Scotland: 28th March 2021
Wales: 12th February 2021
Links to the relevant legislation below:
Northern Ireland: http://www.legislation.gov.uk/nisr/2019/67/made