A British court on Wednesday backed a key patent on Pfizer Inc's (PFE) Lipitor, the world's top-selling drug, preventing India's Ranbaxy Laboratories Ltd (search) from launching a generic version early.

Shares in the world's biggest pharmaceutical company jumped 3 percent to $25 on the New York Stock Exchange on the decision, while Ranbaxy fell 9 percent to $10 in London dealings.

A judgment handed down from the court ruled that the basic patent on the $11 billion-a-year anti-cholesterol medicine, expiring in November 2011, was valid but a more specific patent, running out in July 2010, was not.

Pfizer said the verdict was "an important victory" but the world's largest drug maker will nonetheless appeal the decision on the second patent. Ranbaxy is also expected to appeal, in a process which lawyers said could take around a year.

The British market accounts for nearly 7 percent of global sales of Lipitor (search).

Oppenheimer & Co. analyst Scott Henry said Pfizer had dodged a modest bullet, because UK sales of Lipitor probably account for no more than "two to five cents" per share of the company's annual earnings.

"What was mostly at stake was sentiment. If Pfizer had lost, people might have interpreted that as a precursor to a likely loss by Pfizer of its ongoing U.S. Lipitor fight" against Ranbaxy, Henry said.

"I still believe Pfizer will win the U.S. case, but you can never make any assumptions in a patent dispute," Henry said.

British courts have a reputation for being tougher on patent-holders than those in the U.S. and many analysts have been expecting Pfizer to win in the U.S. even if it failed in Britain.

In giving his judgment, Justice Pumfrey said he had decided to dismiss Pfizer's 2010 patent, covering specific changes to the active chemical ingredient in Lipitor, on the grounds of "anticipation and obviousness."

A Pfizer defeat in the U.S. could lead to generic competition there by 2007, punching a large hole in the group's future earnings.

Uncertainty over Lipitor is one of a number of problems facing Pfizer, whose shares are trading at near 8-year lows.

The company also stands to lose exclusivity on two other drugs -- Norvasc and Zoloft -- by 2007, while demand for its painkiller Celebrex has been hit by concerns it may increase heart attack risk, following the withdrawal of Merck & Co. Inc.'s (MRK) similar drug Vioxx.