Pfizer faces a far more significant threat to Lipitor. Ranbaxy has brought a suit against two Lipitor patents in the United States, where $6.6 billion of the drug's sales are generated. A ruling is anticipated by the end of the year.

Judge Nicholas Pumfrey ruled in the case which was brought by Indian pharmaceutical company Ranbaxy Laboratories Ltd., which wants to release a generic version of the drug in Britain and was was challenging both patents, and by Britian's Arrow Generics Ltd., which was challenging the second patent that was ruled invalid.

Pfizer, the world's biggest drugmaker, said the decision upholding the exclusivity of the patent covering atorvastatin, the active ingredient in Lipitor, until November 2011, was an important victory for scientists.

"This court decision is consistent with the fundamental principle that patent laws exist to support and encourage medical innovators, not undermine them," Pfizer chairman and CEO Hank McKinnell said in a statement.

World's Best Selling Drug

Lipitor is the world's best selling drug, with revenues of $10.86 billion last year. Pfizer said 7 percent of Lipitor sales are generated in the United Kingdom.

New York-based Pfizer said it would appeal the decision on the second patent, which expires in July 2010 and covers the calcium salt of atorvastatin.

Ranbaxy vice president Charles Caprariello said the company would appeal the ruling which upheld the main patent.

Unless the decision about the main patent is overturned, Ranbaxy can't introduce a patent in the United Kingdom until November 2011, Pfizer said.

More Significant Threat

But Pfizer faces a far more significant threat to Lipitor. Ranbaxy has brought a suit against two Lipitor patents in the United States, where $6.6 billion of the drug's sales are generated. A ruling is anticipated by the end of the year.

The US Food and Drug Administration last month approved widening the pool of people who can take the popular cholesterol drug to reduce the risk of stroke and heart attack to many diabetics who have risk factors for heart disease.

Prudential Equity Group Inc. analyst Timothy Anderson wrote that Wall Street is divided on how the US decision on Lipitor's patents will go, but that his legal consultants expect Pfizer to win on the critical active ingredient patent. He noted that because of the differences in US and UK law, it was hard to say whether the UK verdict foreshadows a favorable US decision.

Pfizer shares rose 62 cents, or 2.6 percent, to $24.92 in morning trading on the New York Stock Exchange.