A leading cancer expert has criticised the Prince of Wales for embracing "unproven therapies".
The prince has backed a controversial cancer therapy
Professor Michael Baum of University College London said Prince Charles had "overstepped the mark" by promoting some alternative treatments.
His comments in the British Medical Journal come just weeks after the prince suggested a controversial diet had helped one woman to beat cancer.
A spokesman for the Prince of Wales rejected the criticisms.
Prince Charles told a conference in June that he knew a woman who had beaten cancer after going on the Gerson diet.
This diet involves eating large amounts of vegetables and fruit and having coffee enemas on a daily basis.
The prince told the conference: "I know of one patient who turned to Gerson therapy having been told that she was suffering from terminal cancer, and would not survive another course of chemotherapy. Happily, seven years later she is alive and well."
Professor Baum said there was no evidence that the diet worked.
In an open letter to the prince, he said: "You promote the Gerson diet whose only support comes from inductive logic - that is, anecdote."
Professor Baum said he supported the use of complementary treatments if there was evidence to show they worked.
But he added: "I have no time at all for 'alternative' therapy that places itself above the laws of evidence."
He urged the prince against promoting alternative treatments that have not been proven.
"The power of my authority comes with a knowledge built on 40 years of study and 25 years of active involvement in cancer," he said.
"Your power and authority rest on an accident of birth."
He added: "I do beg you to exercise your power with extreme caution when advising patients with life threatening diseases to embrace unproven therapies."
The Prince of Wales's spokesman denied that he had been promoting the treatment.
"The Prince of Wales is not promoting alternative medicines over orthodox treatments," he told BBC News Online.
"He is simply reflecting the wishes of 80% of cancer patients who wish to use alternative treatments alongside conventional treatments.
"He wants to see more of these treatments available on the NHS and more research into alternative therapies.
"The Prince of Wales is passionate about an integrated approach to healthcare."