MIAMI — Burger King said Friday it will use trans-fat-free cooking oil at all 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.