The 2012 St Leger winner Encke has been put down after suffering a leg injury.
Encke was banned from racing for six months last year after being doped in the scandal centred on Godolphin trainer Mahmood Al Zarooni.
Charlie Appleby, who replaced Al Zarooni as trainer, said Godolphin owner Sheikh Mohammed was "deeply saddened" by the horse's death.
Encke denied Camelot the Triple Crown when a surprise 25-1 winner of the world's oldest Classic at Doncaster.
The horse passed a drugs test after that victory, but in May 2013 it was announced he had tested positive for anabolic steroids.
Encke, aged five, returned to action earlier this year and was third in last month's Irish St Leger behind winner Brown Panther.
Appleby said his horse, who was never out of the first three in nine starts, sustained a hind limb injury during routine exercise and, following complications, could not be saved.
"Encke was a talented racehorse and will be sorely missed as he was one of the yard's favourites," said the Newmarket trainer.
Encke never finished out of the first three in nine starts, and was third in last month's Irish St Leger behind winner Brown Panther.