Top mare Snow Fairy will miss next month's Prix de l'Arc de Triomphe in Paris after being ruled out for the rest of the year with a leg injury.
Ed Dunlop's five-year-old, a dual Oaks winner in 2010, was found to be suffering from heat in a tendon.
Trainer Dunlop said: "She worked yesterday and we were very pleased with her, but last night we found just a little bit of heat in the leg."
He added that Snow Fairy, third in last year's Arc, could race on next year.
"Mrs [Cristina] Patino is keen if possible to think about next season, but she will not be running again this year," he said.
Snow Fairy, ridden by Frankie Dettori, earned her seventh Group One victory after winning the Irish Champion Stakes at Leopardstown earlier this month.
She was expected to race in the Prix de l'Arc de Triomphe at Longchamp on 7 October and then take on Frankel in Ascot's Champion Stakes two weeks later.
Dunlop had also targeted November's Breeders' Cup in Los Angeles for Snow Fairy, who had only recently returned from a serious leg injury suffered when winning the Prix Jean Romanet at Deauville.
Snow Fairy has won a Group One-level race in five different countries.