A group of senior doctors and scientists has stepped up its campaign to stop homeopathic treatment being funded on the NHS. In a letter to primary health care trusts, the seven argue that the evidence for a benefit from the complementary therapy "is equivocal at best, despite many years of research and hundreds of studies".

The letter comes exactly a year after a similar one from a larger group of scientists, including a Nobel prizewinner and six fellows of the Royal Society. Since then, several PCTs have taken their advice, prompting hospitals such as the Royal London Homeopathic Hospital - which provides a range of alternative treatments - to warn that they may be forced to close because of lack of NHS business.

In April, Peter Fisher, personal homeopath to the Queen and clinical director of the Royal London Homeopathic Hospital, wrote an open letter to the monarch, its patron, asking for her support to save the hospital. The Queen reportedly takes 60 vials of homeopathic remedies with her when she goes abroad in case she falls ill.

One signatory of the latest letter criticised Prince Charles' defence of alternative medicine following the first letter. "It has been wholly inappropriate because it is not his role as Prince of Wales to mingle in health politics," said Edzard Ernst, who is professor of complementary medicine at the Peninsula Medical School in Exeter. Another signatory, David Colquhoun, professor of pharmacology at University College London, called homeopathy "crack-pot medicine". The lead author of the letter, Gustav Born, emeritus professor of pharmacology at Kings College London said: "There are still trusts that continue to use these unproven remedies... That is why we have written again to all the PCTs urging them to follow the commissioning example set by others."

David Fish, medical director at the University of London Hospital trust, which includes the Royal London Homeopathic Hospital, said there was a strong demand from patients for alternative treatments. But Hillingdon PCT, which had been spending around £60,000 annually on homeopathy, decided to stop funding the therapy around two years ago. Hammersmith and Fulham PCT and Westminster PCT, which between them spent more than £300,000 on homeopathy treatment last year, also plan to cease the treatment.

However, the scientists behind the campaign insist it is about more than money. "While it may be tempting to dismiss homeopathy expenditure as relatively small across the NHS, we must consider the cultural and social damage of maintaining as a matter of principle expenditure on practices which are unsupported by evidence," they wrote.