The decision, taken by 53 votes to 48 at the FEI's General Assembly in Copenhagen, has caused huge controversy and protests have been led by the sport's leading federations, including Germany and Britain.

 

The Princess has now written to Professor Leo Jeffcott, the former chairman of the FEI veterinary committee, who last week wrote to her to call the decision "premature, ill-considered and seriously retrograde".

 

She claimed the levels were in line with those applied for non-steroidal anti inflammatory (NSAID) use in humans by the World Anti-Doping Agency and had been advised by "experienced national team veterinarians".

 

But criticism of the decision has continued to grow with endurance racing joining those opposing the decision.

 

Their spokesman, Fred Barrelet, a partner in a leading Newmarket practice, said there was already an "alarmingly rising incidence of fractures and other career terminating injuries" in endurance riding, which is a growing sport in the United Arab Emirates, North America and parts of Europe.

 

Breido Graf zu Rantzau, the head of the German federation, has warned the Princess that she has underestimated Europe's anger on the decision.

 

He said: "We [Europe] cannot help but be united...all the tournament organisers are behind us."

 

 

Related stories

November 2009: Aachen to continue zero-tolerance doping policy despite new FEI rules

November 2009: Leading equine vets write letter of protest to Princess Haya
November 2009: Pippa Cuckson - How the FEI managed to sabotage its own new anti-doping programme
November 2009: FEI vice-president unhappy after bute reintroduced to sport
September 2009: Former Met Police Commissioner wants overhaul of anti-doping in equestrian