Some of the few that had been vigorously tested did work, but others did not, he told a briefing on Monday.

Professor Ernst highlighted cancer websites peddling potentially dangerous therapies, and the risk of herbal medicines damaging conventional drugs.

He said: "If it sounds too good to be true, then it probably is. Don't believe ridiculous claims."

Professor Ernst, who is based at Plymouth's Peninusla Medical School, said people should never take complementary therapists at face value.

Instead they should ask for proof of experience, indemnity cover and probe treatment plans more closely before embarking on a course of complementary therapy.

Cancer websites

Scientific evidence for complementary therapies
Ginkgo biloba: Delays the symptoms of dementia, can slow down some circulatory diseases
Hawthorn: Treats mild to moderate congestive heart failure
Garlic: Reduces cholesterol
Feverfew: Good for migraine
Peppermint: Alleviates symptoms of irritable bowel syndrome
An analysis carried out by Professor Ernst's team of 32 popular cancer websites found a extremely variable quality of information on offer.

"It was quite an eye opener and pretty scary stuff," he said.

"A lot of unproven stuff is being recommended to cancer patients.

"Cancer patients, particularly those who are seriously ill, are desperate patients, and desperate patients will cling to any claim and promise that is being made to them.

"Therefore, I think bogus claims for alternative cancer cures are very, very dangerous."

Among the unproven therapies put forward were the use of powdered shark fin, and a preparation based on apricot kernels.

Professor Ernst was also concerned that some sites advised people to stop using orthodox cancer treatments. He said it was certain that some people would have hastened their death by following poor advice on a website.

He called for a seal of approval scheme to highlight sites that gave information that could be trusted.

Heart patients

Professor Ernst also highlighted another study carried out by his team that found some cardiovascular patients were using herbal remedies alongside conventional treatment.

This is despite evidence that some herbal medicines can interact with conventional drugs to reduce their effectiveness.

For instance St John's Wort stimulates the breakdown of the drug Warfarin by the liver, reducing its ability to thin the blood, and raising the risk of a heart attack in patients already at risk.

More worrying still, most of those who took herbal preparations had not discussed the fact with their GP.

It is estimated that there are 40,000 complementary therapists working in the UK. However, many of these are unregulated.

It is thought about 25% of the population use complementary therapies every year.

Professor Ernst said there was strong scientific evidence that some therapies did work.

However, he said there was also evidence to disprove some therapies - and the vast majority had never been subjected to any rigorous scientific analysis at all.

"I don't like the term alternative medicine, because these therapies are not an alternative to conventional medicine," he said.

"Complementary medicine is the best phraseology because some of these therapies do have a role alongside orthodox medicine - although that is not a blanket statement."

A spokesperson for the Research Council for Complementary Medicine said it was currently undertaking a three-year project funded by the Department of Health to test whether 12 complementary therapies could treat cancer