Sir Paul, a strict vegetarian and animal rights campaigner, told the BBC that he had been hesitant to agree to the use of drugs on his wife Linda when he realised they may have been tested on animals.

Lady McCartney died of breast cancer in April this year.

In the letter, 11 top doctors and scientists argued that it was "scientifically and legally necessary to test new medicines on animals".

They said that some of the century's most important medical advances, such as insulin to treat diabetes, polio vaccines, organ transplants and cancer treatments, depended on animal research.

The signatories included fertility expert Professor Lord Winston and leading surgeon Professor Sir Roy Calne, of Addenbrooke's Hospital, Cambridge.

Testing is 'crucial'

Professor Robert Winston: a signatory

The letter says: "There are many areas of medical research in which animal studies remain crucial.

"We are sure that all of us would prefer it if there were no need to use animals in medical research.

"A lot of effort has been put into developing non-animal-dependent research methods but, until we reach the day when animal experimentation is unnecessary, we must avoid holding back medical progress."

The letter recognised that Sir Paul and his wife "took a stand for the welfare and rights of animals".

It goes on to say: "We support animal welfare and understand that we have a responsibility and a duty to ensure that animals are not used in unnecessary experiments, that when animals are used we will safeguard their welfare as much as we can, and that we will continue to develop non-animal methods of medical research and testing."

'I suppose it is necessary'

Sir Paul, who was speaking on the Des Lynam show on BBC Radio 2, said that he had discovered that animal experimentation was widespread.

He said: "Some of it, I suppose, is absolutely necessary when you come down to the final tests before people.

"I suppose a limited thing is unavoidable, but it is very difficult for me to think like that because I favour the rights of the animals."