MIAMI — Burger King said Friday it will use trans-fat-free cooking oil at all of its U.S. restaurants by the end of next year, following in the footsteps of other leading fast-food restaurants.

The world's second-largest hamburger chain said it was already using zero trans-fat oil in hundreds of its more than 7,100 U.S. restaurants nationwide.

Burger King is known for its flame-broiled burgers but uses cooking oil for its french fries and most of its chicken products.

In tests, consumers determined that more than a dozen items cooked in the new oil, such as french fries and hash browns, tasted the same or better than products cooked in the trans-fat oil, the company said.

Miami-based Burger King Corp. said two trans-fat-free oil blends passed tests. If adequate supply becomes available, the U.S. rollout of the oils could be completed sooner than 2008, the company 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.

Critics have said Burger King was taking too long to move toward the healthier oils. The Washington-based Center for Science in the Public Interest sued Burger King in May, saying the company was moving too slowly and had failed to set a definite timetable for removal of trans fats.

In response to the lawsuit, Burger King said in May it expected to begin the national rollout its new zero trans-fat oil by the end of this year.

Among 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 in May 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 in April 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.

Burger King is owned by Burger King Holdings Inc. and operates more than 11,200 restaurants worldwide. About 90 percent its restaurants are owned and operated by franchisees.

Its shares fell 33 cents, or 1.2 percent, to $26.22 Friday.