Michelle Payne will have to pass a fitness test to ride in the Shergar Cup team jockeys challenge at Ascot.

Payne was the centre of controversy when she left a media function for the event earlier than organisers wanted her to, citing illness.

The British Horseracing Authority said she would have to undergo a medical examination and if she was unable to ride, apprentice Hollie Doyle would take her place in the Girls team.

"Michelle Payne has been advised that she will be required to undertake a medical examination at the racecourse tomorrow prior to being permitted to take her declared rides," a BHA statement said.

Payne's brother-in-law, Kerrin McEvoy, is the captain of the Rest Of The World team for the event on Saturday (Sunday AEST).