Michelle Payne must pass a medical exam at Ascot on Saturday in order to take her scheduled rides for the Dubai Duty Free Shergar Cup.
Hollie Doyle was placed on standby on Friday in case Payne has to pull out, after she left Thursday’s media event in London, complaining of feeling unwell.
The Melbourne Cup-winning jockey was also reported to have sworn at an Ascot official when asked to stay longer to be available for interviews.
A BHA statement on Friday confirmed Payne will have to pass the doctor in order to compete in the team-riding event. It read: "The BHA has been advised that Michelle Payne, a declared rider in the Dubai Duty Free Shergar Cup, to be run at Ascot tomorrow, was unwell following her arrival from Australia.
"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."
Should Payne not pass the examination, then Hollie Doyle will ride in her place. The apprentice will be allowed to use her 3lb claim in all races.
Shergar Cup teams
Great Britain & Ireland
Fran Berry, Neil Callan, Jamie Spencer
Alexander Pietsch, Adrie de Vries, Umberto Rispoli
Rest of the World
Anthony Delpech, Kerrin McEvoy, Keita Tosaki
Michelle Payne, Hayley Turner, Emma-Jayne Wilson
Ascot director of racing and communications Nick Smith said: "We hope very much that Michelle will be fit to ride. She is resting up now, and very hopeful.
"We are very grateful to Hollie for agreeing to sit on the subs bench tomorrow, before she goes off to ride at Lingfield in the evening."
Payne has denied swearing and told NewsCorp in Australia she had understood the event would last two hours and not four.
"They wouldn't let me leave," she said. "I was up all night sick and told the lady when we left for the press conference."
Doyle, the season's leading female rider with 35 wins this year, is apprenticed to Richard Hannon. She would be free to join Emma-Jayne Wilson and Hayley Turner in the Girls team as her only booked rides on Saturday are at Lingfield in the evening.
Payne's brother-in-law Kerrin McEvoy, captain of the Rest of the World team, missed the whole media event because he was also feeling unwell. He is expected to take part and walked the track on Thursday afternoon.