Health watchdogs yesterday warned of "a worrying new trend" in viruses that are becoming resistant to drugs.

Research following nearly 4,500 British HIV patients suggests that within six years of starting treatment 27% of them become resistant to at least one drug in the standard triple cocktail they are prescribed.

One in five patients with hepatitis B, which can cause liver disease, is carrying a form of the virus that is resistant to the standard treatment.

Officials at the World Health Organisation and the Health Protection Agency (HPA) in England and Wales are also watching anti-flu drugs, although resistance among the most commonly used treatments is at present running at less than 1%.

Sir William Stewart, chairman of the HPA, said: "Viruses that are resistant to antivirals are beginning to emerge. To a microbiologist this may be unsurprising, but it is worryingly reminiscent of the development of antibiotic-resistant bacteria over 50 years ago."

The triple therapy, which revolutionised HIV treatment in the mid-1990s, may be under threat. But the treatment has reduced the rate of progression from HIV to Aids or death by 86% compared with no treatment, a study by Bristol experts reveals in the Lancet medical journal today.

The cocktail treatment means patients get a drug from each of three different classes, each of which fights the virus in a different way. There are about 20 drugs in all, so when a patient fails to respond to one, another can be tried, but more and more shuffling of drugs is taking place.

Among those in Britain with HIV, about one in 25 is now resistant to at least one drug in all three classes. Similarly, about one in five newly diagnosed patients is proving resistant to at least one drug, and much of that is thought to be caused by having unsafe sex with someone who already is on anti-HIV treatment.