Jockey Joseph O'Brien will replace Frankie Dettori on the 2013 Derby winner Ruler Of The World in the Champion Stakes at Ascot on Saturday.
Dettori had ridden the horse in his last two races, including a fine victory at Longchamp in September.
O'Brien will ride for his trainer father Aidan on Champions Day, Britain's richest raceday.
Nine horses have been declared for the £1.3m feature race, including Cirrus Des Aigles and Free Eagle.
The going at Ascot on Thursday was described as heavy, soft in places after persistent rain during the week.
Officials have said they are confident the meeting will go ahead despite the soggy conditions.
Ruler Of The World is jointly owned by Al Shaqab Racing, founded by Qatar's Sheikh Joaan al Thani, and the trio of Ireland's Coolmore Stud boss John Magnier, Michael Tabor and Derrick Smith.
Dettori is retained rider for Al Shaqab, while O'Brien is stable jockey at his father's Coolmore-backed Ballydoyle yard.
Victory on Saturday for Cirrus Des Aigles, trained in France by Corine Barande-Barbe, would take the horse's total career prize money past £6m.
The eight-year-old gelding won the Champion Stakes in 2011 before finishing runner-up to Frankel and Farrh in the subsequent runnings.
Other contenders in the mile-and-a-quarter contest include Noble Mission, trained by Lady Cecil, the widow of 10-time champion trainer Sir Henry Cecil.
Noble Mission is a younger brother of Frankel, who bowed out with victory two years ago in the race, his 14th win from 14 runs for Sir Henry.
Estimate, owned by the Queen, has been confirmed for the Long Distance Cup alongside Leading Light, who O'Brien trained to win the Gold Cup at Ascot in June.
In the Queen Elizabeth II Stakes, champion jockey Richard Hannon will ride 2000 Guineas winner Night Of Thunder, with Kieren Fallon on stablemate Toormore.