Calandagan landed the big race on British Champions Day after Powerful Glory made history with a 200-1 victory and Cicero's Gift was another shock winner at 100-1.
French challenger Calandagan, ridden by Mickael Barzalona for trainer Francis-Henri Graffard, won the Champion Stakes from Ombudsman at Ascot.
The Richard Fahey-trained Powerful Glory, under Jamie Spencer, beat 2-1 favourite Lazzat into second in the Champions Sprint Stakes.
Powerful Glory is the longest-priced winner of a top-level British Group One race, three months after Qirat set a record by winning the Sussex Stakes at Goodwood at 150-1.
Spencer came with a late surge on his mount to triumph on Britain's richest race day, with 66-1 shot Quinault third in the 19-runner contest.
"I am lost for words," said Spencer.
"Powerful Glory is a good horse and travelled nicely. I thought I was going to be placed and all of a sudden I thought I actually have a chance."
There was another big upset as Jason Watson rode 100-1 chance Cicero's Gift to victory in the Queen Elizabeth II Stakes for trainer Charlie Hills.
A tearful Watson came home ahead of The Lion In Winter and Alakazi to claim his first Group One win since 2019.
