Frankie Dettori will ride Gold Cup winner Order Of St George for Aidan O'Brien in the Prix de l'Arc de Triomphe at Chantilly on Sunday.

Dettori, who will be seeking a fifth win in the 12-furlong feature, had been provisionally booked by trainer Dermot Weld for the 2015 Champion Stakes winner Fascinating Rock on Monday, but his participation was ruled out 24 hours later.

O’Brien’s charge has run twice since his finest hour at Royal Ascot, winning the Irish St Leger Trial but surprisingly beaten at odds of 1 to 7 in the Irish Leger itself.

The Galileo colt will be running over a mile and a half for the first time.

Speaking in his blog as an ambassador for Ladbrokes, Dettori said: “I’ve picked up the ride on Order Of St George and I’m every bit as thrilled as I was when I rode Camelot [for O’Brien] in the race four years ago.

“I was getting a bit anxious that I might miss out on an Arc ride after Fascinating Rock failed to make the race so I am delighted that Aidan has asked me to ride for him again.

“Everyone knows Aidan has had this race in mind for my horse for a while and the record of stayers in the race is good.

“His best form is probably over further but Marienbard had run in a Gold Cup the year before he went on to win the Arc for me, so I know it can be done. I wouldn’t be too worried about the trip.

“Postponed is clearly the biggest threat. It’s difficult to see any flaws in the favourite but my horse is trained by a genius. Now I’ve had the good news confirmed I can relax a bit and start looking forward to a big run.”

O'Brien is also set to saddle Found and Highland Reel, who are expected to be ridden by Ryan Moore and Seamie Heffernan respectively in a race that will see a maximum of 16 runners.

Postponed is unbeaten in four starts since joining Roger Varian, enjoying Group One victories in the Dubai Sheema Classic, the Coronation Cup and the Juddmonte International.