Royal Diamond could miss Sunday's Gain Irish St Leger at the Curragh in favour of a tilt at the Caulfield Cup in Australia next month.
The Johnny Murtagh-trained seven-year-old clinched Group One glory at the Curragh 12 months ago and appeared set to defend his crown on Sunday after winning the Irish Leger Trial over the course and distance in late August.
But due to quarantine restrictions, Royal Diamond would not be allowed to run in the Irish Leger as well as the Caulfield Cup on October 16.
Connections must now make a straight choice between the two.
Royal Diamond's owner Andrew Tinkler said: "No decision has been made yet, but we are thinking about it (running in the Caulfield Cup).
"We're just weighing everything up at the moment and we'll have to make a decision later in the week."
Leigh Jordon, Racing Victoria international recruitment officer, said it would be "a coup" if Royal Diamond was rerouted to the Caulfield Cup, after which he could take in the Melbourne Cup in November.
He told www.racingvictoria.net.au: "Johnny Murtagh, the trainer and rider of Royal Diamond, has advised me that he is now considering a trip to Australia for the Caulfield and Melbourne Cups, rather than defending his Irish St Leger crown this weekend.
"The quarantine requirements mean he would be unable to do both so he will review the acceptances and advise me within the next 48 hours of his decision.
"Certainly it would be a coup for Caulfield if the Irish St Leger winner chose to compete in their race."