All stocks of an Aids drug were tonight recalled across the whole of the UK and Europe after the manufacturers reported that they had been contaminated in the manufacturing process with a cancer-causing substance.

Anybody taking the drug, Viracept, is being asked to visit their doctor immediately, who will take them off it and give them a different medicine.

Viracept is one of the older antiretroviral drugs and many people who were once on it will have been switched to a newer medicine. It is, however, much used in the developing world since the manufacturers, the Swiss company Roche, lowered the price under pressure from campaigners in 2003.

The contamination occurred at Roche's plant in Switzerland, but was detected in Spain where batches of the drug were released for distribution around Europe.

It is understood the contaminant, which is genotoxic and could cause cancer, might have been in the plant for as long as three months - since March.

There are 40,000 people diagnosed with HIV in the UK, but not all are on medication.

A cocktail of three antiretroviral drugs is normally prescribed once the levels of virus in the body reach a certain level and begin to cause symptoms.

The medicines are very effective in suppressing the virus, enabling people to live normal lives.

The Medicines and Healthcare Products Regulatory Authority sent out an alert to doctors last night. "Patients prescribed Viracept should contact their doctor immediately," it said in a statement. "They will have to change to another appropriate medicine for their condition."