Some of the UK's most eminent doctors have mounted a direct challenge to the integration of complementary medicine into the NHS, on the day that Prince Charles urges the World Health Assembly in Geneva to back the cause of alternative and complementary medicines alongside scientifically-proven treatments.

Thirteen senior doctors have written to every hospital and primary care trust in the UK urging them not to suggest anything but evidence-based medicine to their patients.

There has been growing concern among some in medical and scientific circles about the increasing referral by GPs to complementary medicine practitioners. Some GPs use therapies such as acupuncture and homeopathy on their patients; others are increasingly willing to send them to complementary therapists in cases where they cannot themselves provide treatment.

Signatories to the letter include Sir James Black, who won the Nobel Prize for Medicine in 1988, Sir Keith Peters, president of the Academy of Medical Science, and, according to the Times, six fellows of the Royal Society. Yesterday a spokesman for the Royal Society said that it had not organised the letter, but acknowledged that the society took a sceptical view. "As far as the society is concerned, it has always said that alternative medicine needs to be assessed on the same sort of criteria as conventional medicine - but we have not expressed a view about its role within the NHS," said Bob Ward.

The letter was organised by Michael Baum, a cancer specialist who is emeritus professor of surgery at University College London. Edzard Ernst, professor of complementary medicine at Exeter University, is another signatory.

The doctors urge primary care trusts not to spend money on unproven therapies at a time when the NHS is short of cash. It criticises two recent initiatives of the Prince's Foundation for Integrated Medicine - a patient guide to complementary medicine, for which it was given government funds - and last year's Smallwood report, which purported to find that complementary medicine on the NHS was cost-effective.