Now he has infuriated the medical profession by backing a controversial cancer treatment which involves taking daily coffee enemas and drinking litres of fruit juice instead of using drugs. Charles gave an enthusiastic endorsement last week to the Gerson Therapy, which eschews chemotherapy in favour of 13 fruit juices a day, coffee enemas and weekly injections of vitamins.

Cancer specialists have told The Observer that there is no scientific basis for the theory and that it can be dangerous because patients who are seriously ill often come off their normal treatment to try something unproven which may leave them badly dehydrated.

Speaking to a room of 200 healthcare professionals at a conference, Charles said: 'I know of one patient who turned to Gerson Therapy having been told she was suffering from terminal cancer and would not survive another course of chemotherapy. Happily, seven years later, she is alive and well. So it is vital that, rather than dismissing such experiences, we should further investigate the beneficial nature of these treatments.'

Charles's spokesmen last night refused to say whether the patient he referred to is a close friend or someone he met in his role as patron of four cancer charities. What has become clear is that a tight network of friends and associates are advocates of the therapy. Dudley Poplak, an interior designer who has a client list of the great and the good, is the man who is thought to have first alerted Charles to the treatment. Poplak redecorated Highgrove House for Charles and Diana and designed their apartment in Kensington Palace. He gave Charles a copy of the book A Time to Heal: My Triumph over Cancer - Beata Bishop's story of how she beat malignant melanoma 23 years ago by following the strict dietary regime.

Bishop would not comment yesterday on whether the prince had read her book, but said: 'For years the orthodox medical profession has been totally ignorant of the immense potential of nutritional therapy. Finally they have admitted that if you eat the right food you can prevent serious illness, but they still say that if you are ill, you can do nothing. I am not the only one who has survived on Gerson; there are many others like me.' Max Gerson, a German-born physician, gave his name to the rigorous diet, which he initially used to cure his own migraines. He gained a huge following and moved to the US after practising in Paris, but incurred the wrath of the American medical profession for presenting five patients alive and well to a US congressional committee, years after they had been sent home to die.

He believed that cancer was the symptom of a diseased, polluted body in which tumours form when the liver, pancreas and other organs are out of balance, and reasoned that animal and dairy products and other chemicals must be banned. The coffee enemas are used to strip the gut of harmful bacteria and pollutants, but specialists argue they often lead to other problems such as dehydration.

Today the Gerson Institute, run by Max's 82-year-old daughter Charlotte, has an office in California but runs its main clinic in Tijuana, Mexico, because the US forbids doctors to practise it. Treatment costs $4,900 a week and usually lasts for around three weeks.

Another of Charles's associates, the hereditary peer and crossbencher Lord Baldwin of Bewdley, went to the Tijuana clinic in 1996 when his wife Sally was seriously ill with breast cancer. She spent eight weeks at the clinic, followed by another two years of using the regime at home. Her disease recurred and she died three years ago.

Lord Baldwin, who has known the Prince of Wales for many years, invited Charlotte Gerson to address the House of Lords in 1999 and expound her theory. From his home in Oxford, Baldwin said yesterday: 'The subject does arouse passions on both sides but there is an enormous need for proper studies into these treatments. I watched my wife's tumours shrink away but the treatment is a very difficult one to follow. The drugs companies have millions to spend on research, but there's nothing to spend on research into how nutrition might work.'

Charles set up the Foundation for Integrated Health, an initiative to integrate different systems of medicines and therapies, six years ago and has managed to persuade ministers to put millions of pounds more funding into alternative medicine in the NHS. Surveys suggest it is growing in popularity with some 20 per cent of people using an alternative treatment such as acupuncture between two and five times a year.

In his speech, delivered on Thursday in London, Charles argued that it was impossible to separate the mental and physical states of wellbeing, and that the NHS should be developing more funding to different approaches. 'We must commission and produce research that looks at the efficacy of complementary medicine. For example, many patients use and believe in Gerson Therapy, yet more evidence needs to be available as to who might benefit or what the adverse effects might be.'

One of the patients, Frances Carroll from Cheshire, said at the conference she had benefited from Gerson Therapy for the past seven years. Carroll was diagnosed with breast cancer eight years ago and turned to the therapy after she did not appear to respond to conventional treatment. She said: 'It is a means of staying alive. I think it has extended my life expectancy. One might say I've beaten the odds.'

But the American Cancer Society warns that the therapy may be dangerous. On its website it states: 'Gerson Therapy can be very harmful to the body. Coffee enemas have been associated with serious infections, dehydration, constipation, colitis (inflammation of the colon), electrolyte (salt and mineral) imbalances, and even death.'

One prominent British proponent of Gerson Therapy is the Oxford don Michael Gearin-Tosh, who wrote a book about his 'medical mutiny' after he was diagnosed with cancer nine years ago. He converted to the Gerson Therapy and has stuck to the rigorous regime of fruit and vegetables.

Professor Tony Goldstone, head of the North London cancer network, said there was evidence that some patients reached a 'natural plateau' in their disease which could last for years. He attacked the raising of 'false hopes' which 'lead patients in the wrong direction'.

Other cancer doctors pointed out that many units, including Goldstone's, are now offering an array of complementary therapies alongside conventional treatment, because it is popular with patients and because it can be relaxing and mentally helpful for them.

But there is still an enormous lack of evidence, as Prince Charles pointed out, to show what works. One eminent cancer specialist who asked not to be named said: 'If I start to see patients coming off treatment to embrace this therapy, I'm going to be very angry. Charles is abusing his position to put around wacky ideas which have no scientific credence. Imagine the row there would be if Tony Blair did something like this.'

Yet leading cancer specialist Professor Karol Sikora says the role of diet in treating cancer is extremely important and needs more research. 'There is no rationale for the Gerson diet, which is quite radical. Why would a coffee enema work? It's very popular among the higher end of the middle classes, partly because it's expensive and because there's an element of religious mania to it.

'I think that some of what the prince has done in calling for assessment of complementary medicine is a good thing because it can help improve the quality of life. But the idea that huge amounts of Vitamin C can cure you of cancer is simply wrong.'

· Additional reporting by Mark Hudson

The Gerson regime

What is it? A gruelling nutritional regime which would typically consist of 13 glasses a day of fresh organic carrot apple or broccoli juice, vegetarian food, weekly injections of liver extract and vitamin B12, and five coffee or camomile enemas a day. Yoga is also recommended.

How popular is it? An estimated 1,000 people are following it worldwide, but the cost of the injections - more than £20,000 a year - means many cannot afford the treatment.

So what is the theory? Its creator Max Gerson believed cancer was caused by poor nutrition and long-term exposure to pesticides, chemical fertilisers, air and water pollution, and that the right foods and enemas boosted the body's ability to eliminate these toxins - and the cancer.

Is there any evidence for this? There is growing evidence that eating lots of fruit and vegetables has a protective effect against cancer, but studies on the role of food during treatment are sparse. Scientists say there is no evidence that huge doses of vitamin C or enemas can destroy tumours completely.