George Washington was ridden by Kieren Fallon

Holy Roman Emperor had been second favourite behind old rival Teofilo for this year's Classic at Newmarket.

A statement from the Coolmore Stud said: "In the very early stages of George Washington's stud career, his fertility has been questioned."

The horse had been nicknamed 'Gorgeous George' by the breeding operation.

Both horses were trained by Aidan O'Brien at the Ballydoyle stables in County Tipperary, which is part of the multi-million pound Coolmore outfit headed by tycoon John Magnier.

"We were shell-shocked about it here because Holy Roman Emperor was our best horse," said O'Brien.

"It's a a bit like having a team getting ready for the Premiership and your striker's soul being taken away."

The Coolmore statement said veterinary expert Dr Dixon Varner had been brought in from the USA to advise on the matter. "We are awaiting prognosis," it said.

George Washington, a son of leading sire Danehill, was one of the most impressive Guineas winners of recent times when defeating Sir Percy, who went on to win the Epsom Derby.

It's a big shock to everyone here Trainer Aidan O'Brien

He was retired for stud duties as a three-year-old after a career which also brought victory in the Queen Elizabeth II Stakes at Ascot.

The stallion, described by Coolmore as "an outstanding physical specimen", commanded a fee of 60,000 euros (�40,000) a time from owners who wanted their mares to breed with him.

He has been relieved of his stud obligations pending the veterinary report, but it is feasible the horse could return to racing.