MIAMI - Burger King Holdings Inc. said in response to a lawsuit Wednesday that it is testing a cooking oil blend that, if successful, could be used to replace trans fats in its restaurants by the end of the year, earlier than it previously indicated.

Burger King, the world's second-largest hamburger chain, said in January it had begun in-restaurant testing with several trans fat-free cooking oils. At the time, the company said it was on track to begin a national rollout of trans fat-free cooking oils by late 2008.

Based on that, the Center for Science in the Public Interest sued Burger King on Wednesday, claiming the company was moving too slowly and had failed to set a definite timetable for removal of trans fats.

Burger King fired back, saying the chain may be able to complete a rollout of trans fat-free oil in its 7,600 restaurants in the United States and Canada by end of 2007, based on the success of one oil blend it is currently testing.

But if management decides to use a different oil that's also being tested, the change to trans fat-free oil will be delayed until 2008, said Burger King spokesman Keva Silversmith.

The company still pledges to meet trans-fat bans in any municipality, including New York, which has set a July 1 deadline for restaurants to ban artificial trans-fats, Silversmith said.

"The move to trans-fat-free oils is one of the highest priorities for our company," Silversmith said.

Trans fats are listed on food labels as partially hydrogenated vegetable oil. They can raise bad cholesterol and lower healthy cholesterol, increasing the risk of heart disease, doctors say.

Among Miami-based Burger King's main competitors, McDonald's Corp. said earlier this year it had selected a new trans-fat-free oil. Wendy's International Inc. started using cooking oil with zero grams of trans fat in August 2006.

Starbucks Corp. announced earlier this month that it will cut artificial trans-fats out of food and drinks in its stores in the continental United States, Alaska and Canada by the end of the year.

Yum Brands Inc. said last month that all of its KFC restaurants are now serving fried chicken with zero grams of trans fats. Yum Brands also said its Taco Bell restaurants switched to a trans fat-free frying oil.

The centre's lawsuit, filed in Washington, called for Burger King to cease its use of trans fats in a timely manner, or at least order the company to inform customers that it uses trans-fats.

Michael Jacobson, the centre's executive director, said it would be "terrific" if Burger King were able to go trans fat-free by the end of 2007, but he remained skeptical.

"It could be pure PR," Jacobson said. "It's hand-waving, because a week ago they weren't talking about this at all. If they had given a firm commitment by last week, there wouldn't have been a lawsuit."