The career of sprinter Takeover Target is over after he fractured a cannon bone in the July Cup at Newmarket.
The 10-year-old finished seventh behind Fleeting Spirit but it became clear after the race that he had a problem.
Trainer Joe Janiak said: "He's had five screws inserted and will be box rested at the hospital for a couple of months.
"He seems fine after the operation and he needs to rest. He has been a fantastic horse and we have had some great adventures with him."
Janiak famously bought Takeover Target in 2003 while he was still a taxi driver.
He won eight races at the highest level and was a regular visitor to Royal Ascot, although he had to miss this year's meeting because of injury.
He retires with 21 career wins and earnings of more than £6m.
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