The world's biggest software company, Microsoft, and the pharmaceutical multi-national Pfizer are taking legal action against individuals and companies who, they allege, market and sell the anti-impotence drug, Viagra, without Pfizer's consent.

It is alleged that counterfeit versions of the drug are being sold on the Internet from the US, Canada and India.

Pfizer says the copies could be dangerous and also deprive it of profits.

Reports say hundreds of millions of e-mails offering the drug have been sent in the past year to people on the Microsoft e-mail system, Hotmail.