William Muir and Chris Grassick have been dealt a hammer blow with stable star Pyledriver ruled out of the King George VI and Queen Elizabeth Qipco Stakes on Monday morning following a setback.
The four-year-old had worked well on Saturday morning under regular rider Martin Dwyer but a minor issue has since surfaced, forcing connections to rule him out of Saturday's big race at Ascot, for which he was a general 5-1 chance.
Pyledriver had previously provided joint-trainers Muir and Grassick with a landmark first Group 1 winner when he bravely held off the challenge of Al Aasy to win the Coronation Cup at Epsom last month, having been beaten on his first start of the campaign in the Jockey Club Stakes at Newmarket.
"Saturday was unbelievable, we'd done exactly the same as we did for Epsom," said Muir. "He did a piece of work with Martin who said it was magnificent. The way he came through was fantastic and I walked away from the gallops thinking it will take a good horse to stop us.
"I met him coming back along the road and he was bouncing, bobbing and kicking, and I thought this horse could have not been in better shape. I got a phone call from the evening stables when I was at Newbury on Saturday from my vet saying he thought he may have tweaked his right groin.
"I told the owners you can't have any setback, you cannot run in that type of race. I've seen things all my life in racing when you try to do something and it all goes wrong, I couldn't live with myself if that happened. I can't take a chance with any horse."
The La Pyle Partnership-owned son of Harbour Watch enjoyed a memorable season last year, providing Muir with his first Royal Ascot victory in the King Edward VII Stakes as well as landing the Great Voltigeur Stakes at York's Ebor festival.
Pyledriver could now be aimed at a return to the Knavesmire for the Juddmonte International next month before a prolific autumn campaign, with options including the Prix de l'Arc de Triomphe, Breeders' Cup, Hong Kong and the Japan Cup.
Muir added: "The vet will come today and give him the full once over and then we'll go from there. If it's something very simple and he's fine in three days then he'll go for the Juddmonte, which he definitely has the speed for.
"If it's something we have to give him four or five weeks off for and have a break, we've got so many options to campaign him towards still.
"I thought he'd win on Saturday, I really did, but we're not going to be part of it anymore, it's tough. I'm gutted to be honest."
His defection is a big blow for the 1m4f Group 1, in which the best of the generations are expected to clash with Prince of Wales's Stakes winner Love and Saudi Cup hero Mishriff potentially poised to take on Derby victor Adayar.
Ascot is expecting to have the biggest crowd on a British racecourse in 16 months for Saturday's King George, with the track expecting upwards of 14,000 racegoers for the "race of the season".