The 48-year-old Yorkshireman and the horse, Portofino 63, are now on their way home from Hong Kong, where the equestrian events are being held.

 

They will be replaced by 50-year-old Nick Skelton and Russel, a grey stallion.

 

A statement from the British Olympic Association said: "The team veterinarian has carried out an extensive investigation but as yet has been unable to target the source of the problem. 

 

"Portofino...was sound during pre-export quarantine in the UK and on arrival in Hong Kong but has developed a problem in the last 48 hours. 

 

"It has been decided in the horse's best interests to withdraw her before the Horse Inspection."

 

It is a blow to Whitaker, who won a silver medal in the team event when he made his Olympic debut in 1984 when his team-mates included Sebastian Coe, now the London 2012 chairman.

 

It would also have been the fifth occasion that he has partnered his brother John in the Olympics.

 

It is the second blow for the Whitaker family.

 

In June Michael's niece Ellen had to withdraw from Olympic consideration because of an injury to her horse.

 

It will be Skelton's fifth Olympic appearance having competed in every Games since his debut in Seoul in 1988, with the exception of Sydney 2000.

 

In Athens four years ago, riding Arko III, Skelton went into the final round of the competition with the gold medal in his sights but had three fences down and slipped to 11th.