Pfizer, the world's biggest drug company, has sued four of its rivals in an attempt to protect its famous erectile dysfunction drug, Viagra.

US firms Eli Lilly and Icos are planning an impotence remedy known as Cialis.

Britain's GlaxoSmithKline and Germany-based Bayer want to launch Levitra into the same market.

But Pfizer, which has just taken out a new patent on Viagra, claims the new drugs infringe its intellectual property.

Generic controversy

Cialis and Levitra are half-way through the approval process of the US Food and Drug Administration, and were timetabled for launch next year.

Eli Lilly said it would vigorously defend itself against the Pfizer lawsuit.

Bayer said it, too, would contest the lawsuit but it said it was confident that the legal action would not delay the US launch of Levitra.

The row over Viagra lookalikes is the latest twist in an increasing controversy over so-called generic drugs - remedies that are copies of existing brand-name products, while undercutting them on price.

Consumers in the US are keen to gain more access to generics, which are often blocked by major manufacturers.

Dozens of drug cases

Earlier this week, US President George W Bush promised to open the market further, in the hope of providing a wider choice of drugs, especially for the elderly.

The Viagra case is unusual, because the firms making the generic drugs are also major manufacturers.

In most other cases, makers of generic drugs tend to be smaller companies, often from the developing world.

However, the makers of the Viagra rivals insist that their drugs are not generic copies, but drugs in their own rights.

There are currently dozens of cases relating to generic drugs proceeding through the US courts.