The first thing to do is identify which organisation issued the original passport. If you have a microchip number check it at https://www.equineregister.co.uk/chipchecker
If the passport was issued in the UK you should be able to find it on the above website and you will need to contact the passport issuer that is listed.
If the passport was issued abroad you will probably need to apply to a UK passport issuer to get the duplicate. We can normally help if the original passport was a basic identity passport but if pedigree was recorded you will need to contact an equivalent UK Breed Society.
If you are unable to identify who issued the original passport you can apply for a Replacement passport (see menu).
A duplicate passport by law will be stamped as duplicate and also have to be signed as not for human consumption.
Please visit the Lost Passports section for more information and an application form.