Darley July Cup (G1) winner Starman will miss the QIPCO British Champions Sprint Stakes (G1) and has been retired to stud.
He suffered a minor setback while being prepared for Saturday's Ascot showdown and will not recover in time to take his chance.
His racing career is now at an end and he will stand at Tally-Ho Stud in Ireland, with owner David Ward retaining a share in the 4-year-old for his stallion career.
Trainer Ed Walker said, "It's a huge shame as we were really looking forward to Saturday and hoping he could prove himself a real champion.
"I've always thought he was the best around and he's certainly the best I've trained."
Starman's absence throws Saturday's Champions Sprint wide open as he had topped the betting across the board at a best-priced 4-1.
COVID-19 meant that the colt, who was home bred by Ward, did not make his first appearance until July of last year, but he won on his debut at Lingfield then broke the track record at Doncaster and took the listed Al Basti Equiworld Dubai Garrowby Stakes at York last September.
After being beaten on very soft ground in the Champions Sprint at Ascot the following month, Walker was determined thereafter to avoid running in similar conditions.
That ruled him out of the Diamond Jubilee Stakes (G1) at Royal Ascot this summer, for which he had been a major fancy after his impressive comeback win in the Duke of York Clipper Logistics Stakes (G2) at York in May.
But his top-level breakthrough was merely delayed and the following month Starman powered to a clear-cut success in the July Cup at Newmarket, where he gave Walker his first group 1 win as a trainer.
The 4-year-old went on to finish third in the LARC Prix Maurice de Gheest (G1) at Deauville, seemingly outstayed on very soft ground over 6 1/2 furlongs, and was then beaten just a short head by Emaraaty Ana in the Betfair Sprint Cup (G1) at Haydock on what turned out to be his final appearance.
The winner of five of his eight starts, with earnings of over £476,000 in prize money, Starman retires as the top-rated sprinter in Europe.