NEW YORK (CNNMoney.com) - McDonald's french fries contain one third more trans fat than previously reported, according to a published report.

The Financial Times said that McDonald's revealed a new reading on the level of the potentially harmful substance under new U.S. government rules on food labels to specify the level of the trans fat in food products.

McDonald's has upped its estimates for the trans fat in its french fries by one-third, according to a published report.

The newspaper reports that the company is now reporting that a larger order of its french fries have eight grams of trans fats, rather than the previously-reported six grams.

"It makes it harder to trust McDonald's if they suddenly have strikingly different (trans fat) numbers," Michael Jackson, the executive director of the Center for Science in the Public Interest, told the Times.

Cathy Kapica, the company's global direction of nutrition, told the newspaper that the new reading is because the company is continually improving its testing methods.

"We promptly updated the information on our Web site. We will continue to provide our customers with the most current, accurate nutrition information possible," she said.

There is no "recommended" safe level of trans fat. The independent Institute of Medicine sent a letter to the FDA during its rule making on trans fat in which it "recommend(s) that trans fatty acid consumption be as low as possible while consuming a nutritionally adequate diet."

Trans fats are created when hydrogen is added to vegetable oil in an effort to extend the shelf-life of products such as cooking oils. The Food and Drug Administration says trans fats can cause a rise in so-called "bad cholesterol" and increase the risk of heart attacks. It estimates that its new rules on labeling will encourage Americans to reduce their intake of trans fats and help prevent 600 to 1,200 heart attacks annually within three years.

The six grams of trans fats previously reported by McDonald's was already slightly above the FDA's estimates for the average daily intake of trans fats in the average adult American's diet.

The company has been under scrutiny for its use of trans fats, and in 2002 promised to cut by half the trans fats in its french fries.

But in 2004 a public interest group, BanTransFats.com, sued McDonald's, saying it had broken that promise. McDonald's settled that suit a year ago, agreeing to spend $1.5 million notifying customers that it had not changed its oil to one lower in trans fats, and agreeing to donate $7 million to the American Heart Association for education on the danger of trans fats, in addition to agreeing to pay plaintiffs' attorney fees.

McDonald's french fries are among its products highest in trans fats. The only products on its Web site listed as having more than eight grams of trans fats are its deluxe breakfast with 11 grams and the 10-piece chicken select strips, which has nine grams. By comparison, its signature Big Mac hamburger has 1.5 grams of trans fats, according to its Web site.

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