A POLIO vaccine that was administered to millions of Britons in the 1950s and early 1960s is being linked to a virulent form of lung cancer, following the discovery that it was contaminated by a monkey virus.

Doctors have expressed concern at latest studies in America where the virus, SV40, was found in several tumours. It is estimated that between 10 million and 30 million Americans were given the polio vaccine containing the virus. It is not known how many people in Britain were similarly infected, although several million were given shots between 1955 and 1961, when the vaccine was largely abandoned.

The vaccines contaminated with SV40 contained a protein which stops the body's ability to fight off tumours, according to research by Michele Carbone at the National Institutes of Health in Bethesda, near Washington. She and her team believe that people infected are more vulnerable to mesothelioma, a virtually incurable form of lung cancer that is normally associated with exposure to asbestos.

The vaccine, made from monkey kidney cells, was used in mass vaccinations until 1961. After that a new vaccine using human cells was developed. Vaccines that were still made with monkey cells were subjected to sophisticated tissue culture checks which ensured they were virus-free.

Dr Philip Minor, head of virology at the National Institute of Biological and Standards Control, said scientists here were relying on population studies from America to determine the link between the cancer and SV40. "The findings are causing a flutter but there is nothing we can add to what the Americans are already doing," he said.

Mesothelioma is a malignant tumour affecting the membrane that lines the chest cavity and covers the lungs. Between 300 and 400 people a year are diagnosed with it, with about 80 per cent of cases linked to exposure to asbestos. Until Dr Carbone's study, scientists were puzzled by the number of people developing the cancer who had not been exposed to asbestos.

Dr Minor said it had been shown that there was a link between SV40 and tumours in humans but this was the first time it had been suggested that the polio vaccine could be responsible.

Dr Gordon McVie, director general of the Cancer Research Campaign, said that mesothelioma was a very slow-growing cancer, often taking at least 20 years before it became detectable.

The incidence of mesothelioma was rising, he said, and while most cases could be attributed to earlier exposure to asbestos, there was a question mark over 20 per cent of cases.