Aidan O'Brien has issued concerns over the potential fitness of Australia as the dual Derby winner is pitched into all-aged competition for the first time in the Juddmonte International at York tomorrow.

Since being narrowly beaten into third by Night Of Thunder and Kingman in the 2000 Guineas, the son of Galileo and Ouija Board has blown away his opponents at Epsom and the Curragh to confirm himself the leading three-year-old over middle distances.

The trainer's son Joseph is set to slim down to 8st 12lb to keep the partnership intact.

This, though, is the likely odds-on favourite's first outing since winning the Irish Derby on 28 June and his team have later targets in mind.

Aidan O'Brien said: "He has done very well and is just ready to start back again.

"I suppose, because he has done so well, we are a little bit worried about his fitness.

"But really, when you give a horse a break - especially a three-year-old - going into the autumn, you want him to do well. He did well, he did very well."

However, just two older horses have turned up, with the Michael Stoute-trained Telescope and William Haggas' stable star Mukhadram both proven top-notchers, having finished second and third respectively in the King George at Ascot last month.

Mukhadram had previously claimed a first Group One in Sandown's Coral-Eclipse, while Telescope impressed in the Hardwicke Stakes at the Royal meeting.

Kevin Ryan's French Derby winner The Grey Gatsby is an interesting contender, returning to the scene of his victory in the Dante Stakes earlier in the year.

Peter Chapple-Hyam's Arod has ground to make up on Australia on Epsom form, but is expected to be more effective over a mile and a quarter.

Australia's stable companion Kingfisher completes the field.