Owners of equines in the UK must have their equines microchipped.
Previously, owners of equines born before 30th June 2009 did not have to have a microchip fitted, but the law changed to make it compulsory for all horses to be microchipped, regardless of age

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 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:
​England: https://www.legislation.gov.uk/uksi/2018/761
​Northern Ireland: http://www.legislation.gov.uk/nisr/2019/67/made
​Scotland: http://www.legislation.gov.uk/ssi/2019/30/made
​Wales: http://www.legislation.gov.uk/wsi/2019/57/made