The two-year-old, who had won his last three starts over six furlongs, had been pencilled in for a run at next month's Breeders' Cup.

Successive Group One victories in the Prix Morny at Deauville and the Middle Park Stakes at Newmarket had raised hopes for the No Nay Never colt.

In fact O'Brien recently claimed he was targeting the Breeders' Cup Juvenile Turf Sprint at Keeneland for the horse.

"We have Blackbeard for the Sprint and Aesop's Fables is also possible for that," said Aidan O'Brien.

"He's a very unusual horse, it's unbelievable what he has done this year. He's been busy and has stood up to everything. The busier he's got, the better he has got."

After winning six of his eight starts for O'Brien, there were high hopes for the colt, but he was found to have chipped a knee and time has been called on his racing career.

"Blackbeard was a little off after exercise this morning in preparation for the Breeders' Cup. Unfortunately, when we had him X-rayed the radiographs showed a small chip in his right knee and a decision has been made to retire him," O'Brien said on the Coolmore website.

"He's a typical No Nay Never; strong, early maturing and very fast. He ran every month from April to September and improved and improved from one race to the next.

"The plan is for No Nay Never's other brilliant son Little Big Bear to be trained for the Guineas along with Saturday's winner Auguste Rodin, while Luxembourg also remains in training for next season."

Ryan Moore, who partnered Blackbeard in all but one of his races, said: "Blackbeard was a very exciting horse to ride. He was tough as nails and the ultimate professional who continued to improve throughout the season and had a lethal turn of foot."