(Adds Roche comment, further details on drug)

LONDON, June 6 (Reuters) - Roche Holding AG's <ROG.VX> HIV drug Viracept is being recalled from markets in Europe and some other countries due to contamination, European authorities and the Swiss drugmaker said on Wednesday.

"There is a problem in manufacturing," said a spokesman for the European Medicines Agency, explaining the sudden decision.

Patients taking Viracept, a first-line HIV therapy, are being urged to contact their doctor and will have to change to another appropriate medicine.

The UK's Medicines and Healthcare products Regulatory Agency (MHRA) said there had been contamination with a genotoxic substance, one which can affect the genes and cause cancer.

Roche said it had identified a chemical impurity in the product, leading to the recall in Europe and "some other world regions".

The problem does not affect forms of the medicine sold in the United States, Canada or Japan, it added.

"Roche has received several reports that some batches of Viracept 250 mg tablets have a strange odour," the company said in a statement.

"A detailed chemical analysis of the affected tablets showed they contain higher than normal levels of methane sulfonic acid ethylester. In the interest of patients' safety, Roche has decided to recall all batches of Viracept tablets and powder."

Viracept, known generically as nelfinavir, belongs to a class of HIV/AIDS drugs called protease inhibitors. It forms a key part of many drug cocktails used to treat people when they first need antiretroviral therapy.

Such drugs can slow the spread of the virus in the body and cut the risk of patients developing AIDS-related illnesses.

Viracept received marketing approval in Europe in 1998. It was cleared for use a year earlier in the United States, where it is sold by Pfizer Inc. <PFE.N>.

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