Frankie Dettori will sensationally partner Camelot in the Prix de l'Arc de Triomphe at Longchamp on Sunday.
With trainer Aidan O'Brien's son, Joseph, unable to make the weight of 8st 11lb to ride Ballydoyle's 2000 Guineas and dual Derby winner, Dettori takes over.
Joseph O'Brien will instead ride stablemate St Nicholas Abbey.
Camelot's participation in the Arc had been up in the air since his defeat in the St Leger at Doncaster last month, when he was prevented from claiming the Triple Crown by Encke.
But after having worked at O'Brien's Ballydoyle stable on Wednesday morning, the decision was made to let the three-year-old colt take his chance.
"We were pleased with the work of both St Nicholas Abbey and Camelot this morning in Ballydoyle and hope to run both in the Arc on Sunday," read a tweet from Ballydoyle.
"St Nicholas Abbey will be ridden by Joseph and Camelot will be ridden by Frankie Dettori."
Joseph O'Brien partnered Camelot in his key final Arc prep and said: "I was very happy with him - he did what he had to do."
Dettori, who has ridden in 24 consecutive Arcs, was without a mount in the great race and is a rare sight on O'Brien runners.
He did, however, win the St Leger at Doncaster for Coolmore aboard Scorpion in 2005.
The Italian has also guided Lammtarra (1995), Sakhee (2001) and Marienbard (2002) to victory in Europe's premier middle-distance contest.
Camelot adds a much-needed touch of quality to an Arc which has recently seen three high-profile defections.
Injury forced Snow Fairy out of contention at the weekend, while last year's winner Danedream is unable to travel to France as her stables at Cologne racecourse were on Monday placed into quarantine due to a serious outbreak of an equine infection.
The John Gosden-trained Nathaniel was then declared a non-runner on Tuesday after returning unsatisfactory blood tests when found to have been suffering with a temperature.
There had been rumours Danedream could yet be supplemented back into the race on Thursday at a cost of 100,000 euro owing to some sort of mix-up at Cologne.
However, Patrick Barbe, racing manager to her part-owner, Teruya Yoshida, told racinguk.com: "It is only a rumour.
"There is a no-go zone a kilometre around the training base, I just can't see us being allowed to go."
In addition to Nathaniel, Imperial Monarch was taken out at the latest forfeit stage for the race.
O'Brien had mentioned the Canadian International at Woodbine as being the long-term target for Imperial Monarch and that would now appear his next port of call.
Sixteen horses remain in the all-aged middle-distance championship.
Saonois, the French Derby and Prix Niel winner, is expected to be supplemented on Thursday.
