Hundreds of people have been diagnosed with a strain of HIV that is resistant to all of the commonly prescribed drugs, a report reveals.
Over 19,000 people are being treated for HIV in the UK
Figures from the Health Protection Agency also suggest the number of people whose treatment is failing in this way may be increasing.
In 2001, 154 people were told they were resistant to the three available classes of drugs. In 2000, it was 115.
Officials said the figures highlight the need for new anti-HIV drugs.
There are currently four classes of anti-HIV drugs. Up until recently, there were just three.
The fourth class became available earlier this year with the launch of Fuzeon, which stops the HIV virus from entering healthy cells.
HIV patients are usually prescribed a combination of three different drugs in an effort to control the virus.
They may take two drugs from one class and one from another. This makes it harder for HIV to mutate and beat the drugs.
Doctors generally only test those they suspect of having a resistant strain for resistance.
In 200, just 4% of the 19,312 patients receiving treatment were tested for resistance.
Of these, 22% had no level of resistance. However, 25% were resistant to one drug, 40% to two drugs and 13% to three drugs.
Dr Barry Evans who works at the Health Protection Agency said officials were worried about the growing number of people who are becoming resistant to drugs.
But he added: "Of greatest concern, however, are the number who are resistant to all three available classes of drugs.
"This percentage has increased from 1% in 1996 to 14% in 2001. This reflects the ability of the virus to mutate and replicate despite the presence of drug treatment."
Dr Evans said the figures highlighted the need for more effective drugs to control the virus and for people to be warned about the risks.
"There is still a need to continue to develop new classes of HIV drug treatments, in order to effectively treat people who are experiencing treatment failure.
"New diagnoses of HIV infection in the UK continue to rise and safe sex messages targeted at people living with HIV are essential in order to prevent onward transmission of drug-resistant strains of the virus to newly-infected individuals."
The National Aids Trust also called for more research and better drugs.
"These figures emphasise the need to develop new treatments and to ensure people with HIV have the best information and advice available," said Derek Bodell, its chief executive.
"The increasing drug resistance means we need more choices.
"It also means that people who are at risk of being exposed to the virus understand the implications of infection with a drug resistant virus."