During his annual visit to the Sue Ryder Home in Cheltenham, Gloucestershire, he asked Karen Drake, who has MS: "Have you tried taking cannabis? I have heard it's the best thing for it."

Mrs Drake, 36, said afterwards: "I was surprised but I think I would like at least to try it. Anything that can help relieve the pain can only be for the good."

The Prince raised the subject after she gave him a copy she had made of a Monet painting. A spokesman for the Prince said that the conversation had been private. "If the Prince does have a view on this matter, he is not making it known," he said.

Gillian Rose, the home's appeals co-ordinator, said: "I only presume he mentioned it because it is a talking point for MS sufferers."

In November the House of Lords Select Committee on Science and Technology recommended that doctors should be allowed to prescribe cannabis to some of the 85,000 MS sufferers in Britain. The Government said that it would not consider doing so until there had been extensive clinical trials.

Next month the Multiple Sclerosis Society will publish a protocol worked out with the Royal Pharmaceutical Society on how clinical trials on cannabis and its derivatives should be conducted.

The society believes that trials are essential to find out whether cannabis has dangerous long-term effects and believes that only neurologists should be allowed to prescribe the drug.

"We recognise that some people who use cannabis to relieve symptoms find themselves on the wrong side of the law but we do not feel they should be treated as criminals," a spokesman said.

"There is anecdotal evidence that smoking cannabis can help in some cases but not in all. We wouldn't recommend that anyone breaks the law."