It found that the more assiduous these patients were at taking their medication, the faster the virus became drug-resistant.

The research did not say taking medication as instructed causes HIV to become drug-resistant.

HIV is thought to mutate in people who fail to take their pills, and some doctors deny anti-retrovirals to patients who are unlikely to take their drugs properly.

Drug-resistant HIV

A growing number of people who contract HIV are already resistant to some of the existing drugs used to fight the disease.

This makes it increasingly difficult for doctors to treat them.

This does not mean patients should take less of their drugs to avoid creating resistance Dr David Bangsberg

A study, published in July, found one in 10 new cases of HIV in Europe were resistant to at least one key drug.

Dr David Bangsberg and colleagues at the University of California San Francisco looked at 148 patients taking anti-retrovirals. They were all homeless, living in shelters or B&Bs.

All had a detectable viral load - which means the medication they were taking was insufficient to control their virus.

The researchers carried out spot checks on each of these patients every three to six weeks over the course of one year. They counted their pills to see if they were taking their drugs properly.

They also carried out blood tests to measure their HIV levels and to see if the virus was mutating.

The researchers found that patients who took 80% or more of their pills were twice as likely to be carrying a mutated form of the virus compared to patients who took 40% of their drugs or less.

The mutations were new and had occurred during the course of 12-month study.

Treatment

Dr Bangsberg described the findings as surprising. But he said HIV patients on anti-retrovirals should continue taking their medication.

"What is surprising is that what we typically take to be excellent pill-taking - 80% of pills or better - leads to more resistance than occasional or inconsistent-pill taking," he said.

"This does not mean patients should take less of their drugs to avoid creating resistance.

"Good adherence to the drug regimens still is the best bet to prevent becoming ill or dying with HIV/Aids.

"Many patients with excellent, even perfect pill-taking are living longer with resistant virus than those who do not take medication to select for resistant virus."

Dr Bangberg said the findings could lead to changes in the way doctors treat HIV patients who are regarded as unlikely to take their drugs properly.

"These findings will make us rethink the argument that life-saving anti-retroviral drugs should be denied to some populations because poor pill-taking behaviour might accelerate the creation of resistant mutations of the HIV virus," he said.

Keith Alcorn, a senior editor at the National Aids Manual, told BBC News Online: "This shows people need to be monitored regularly and treatment needs to be changed quickly if it fails.

"Otherwise, people would be doing what they think is the right thing by taking as much medication as possible, but which may make the situation worse."