The new strains "could represent an important emerging clinical and public health problem," according to Frederick Hecht, the author of one of the studies, from the University of California.

Triple drug combinations have revolutionised treatment of Aids. Since their development two years ago the rate of Aids deaths in the developed world has slowed.

The California study to be published in the New England Journal of Medicine found a man who was resistant to six of the 11 approved anti-HIV medicines used in combination therapy.

It seems that he picked up the infection from a sexual partner whose long-term treatments with anti-Aids drugs had allowed the virus to develop resistance.

The spread of resistant strains of HIV has been reported before, but this is the first evidence of the spread of a strain resistant to the class of drugs known as protease inhibitors - one of the most powerful new weapons against the virus.

Dr Anthony Fauci, the director of the US Government's National Institute of Allergies and Infectious Diseases, says this is a "wake-up call."

"If you happen to get infected from a person who has a multiple drug resistant virus, it's just as if you got infected in 1983, when we had absolutely no anti-retroviral drugs."

He also expressed concern that there is mounting evidence of side-effects when people use these drugs for more than 10 years.