You should not purchase a horse without a passport (for full details on the current passport regulations visit the Defra website). Check that the passport silhouette matches the horse you are going to purchase.
It is a legal requirement that the passport is transferred with the horse and given to the new owner at the time of the sale. Do not fall victim to excuses for a passport not being provided, as you will also require a passport for transporting the horse to a new yard. It is then the responsibility of the new owner to send the passport and any required paperwork to the relevant Passport Issuing Organisation to amend the ownership details. Under the Horse Passport Regulation 2009, the passport should be sent for change of ownership no later than 30 days after the sale. Failure to change the ownership is an offence.
Find out more information about horse passports.