David Beckham has been warned to avoid becoming a "pawn" in the Spanish bullfighting industry following his transfer to Real Madrid.
Bullfighting bosses may try to exploit Beckham's superstar status
An animal rights group said bullfighting authorities often invited players to attend bullfights, meet top matadors or visit bull-breeding ranches.
Tony Moore, of the Fight Against Animal Cruelly in Europe, has written to Beckham saying he should guard against the bullfighting industry exploiting his fame.
Mr Moore, of Southport, Merseyside, lost his wife Vicki when she died after being gored by a bull while trying to stop a bull-baiting fiesta in Spain in 1995.
She died as a result of complications caused by the injuries.
Mr Moore said: "There is nothing the bullfighting industry likes more than to receive the apparent blessing of a football star.
Bullfighting is big business in Spain
"David Beckham is the most famous football star of the moment and I have no doubt they will try to get him on board.
"He doesn't even have to go to a bullfight. All he has to do is shake a matador's hand or visit a breeding ranch and, as far as they are concerned, they will have his seal of approval.
"Shortly after Zinedine Zidane signed for Madrid, he was taken to a breeding ranch for the same reason.
"I am warning David Beckham to be on his guard. He has a reputation as a great, honest, hardworking athlete and it would be terrible if he was used as a pawn by the bullfighting industry."
Mr Moore said he had sent a letter to the former Manchester United star and his wife, Victoria, to outline his concerns.
The letter warns that bullfighting "is not a fair game like football".
It adds: "The result is known in advance and always ends with the death of the bull."