By Pallab Ghosh
BBC News science correspondent
Some homeopathic practices tell people they need not take conventional anti-malaria drugs in high-risk parts of the world, an investigation by BBC2's Newsnight has revealed.
Instead, the clinics say that their remedies are sufficient to protect against malaria.
Each year, two million Britons travel to parts of the world where malaria is rife. About 2,000 of them return having contracted the disease.
In the vast majority of cases, they have fallen ill because they have not taken any anti-malaria tablets.
But doctors have begun noticing some cases where patients have taken homeopathic remedies instead of licensed medicines.
Some have claimed that they were told that the homeopathic protection could be used instead of conventional medicine.
Dr Ron Behrens runs a travel clinic at the London Hospital for Tropical Diseases. He has seen several patients who thought they were safe because they were taking homeopathic remedies to protect them.
"We've certainly had patients admitted to our unit with falciparum, the malignant form of malaria, who have been taking homeopathic remedies - and without a doubt the fact that they were taking them and not effective drugs was the reason they had malaria," he says.
According to Dr Behrens, some homeopaths are offering Britons travelling to the malaria belt an easy option and false hope.
"Sub-Saharan Africa is a high risk for malaria. If they got it and they weren't immediately diagnosed and treated, they could die and that claim would actually put their lives at risk," he Dr Behrens.
Such cases prompted the scientific campaigners Sense About Science to send an undercover researcher into 10 homeopathic practices.
She said that she was about to go to a malaria-infested country. They all recommended doses of homeopathic remedies.
These were 99.99% water with an almost undetectable trace of quinine.
Newsnight followed up the research with a hidden camera. A programme researcher told a South London practice she would be travelling in Africa for three or four weeks, including high risk areas such as Malawi.
She said she did not like the side effects of the drugs the doctors prescribe, and asked if there was anything else she could take.
This is the advice she was given by one homeopath: "The doctors have this big fear thing about malaria... obviously it is a nasty thing, but actually, as I say, you can prevent it with the remedies; and as you say, the tablets are really horrible. They're very nasty, have nasty side effects, and I've seen quite a lot of patients who have had serious problems from them."
In that 20-minute consultation, there was no mention of going to the doctor - the homeopath made the claim that malaria could be prevented by taking homeopathic remedies.
When Newsnight called the clinic asking why its consultant gave that advice, it said that this was a mistake. Normally, the clinic said, practitioners did tell people to go to their GP for serious conditions such as malaria.