WASHINGTON (Reuters) - Trans-fats, which are being phased out of food because they clog arteries, may raise the risk of getting breast cancer, European researchers reported on Friday.

They found that women with the highest blood levels of trans-fats had about twice the risk of breast cancer compared to women with the lowest levels.

"At this stage, we can only recommend limiting the consumption of processed foods, the source of industrially produced trans-fatty acid," the researchers wrote in the American Journal of Epidemiology.

Trans-fats or trans-fatty acids are made in creating artificially hardened fats -- in the process of hydrogenization, for instance.

They were, ironically, meant to be healthful replacements for artery-clogging saturated fats such as butter and lard.

But the process of making vegetable oil behave like butter made it as unhealthful as butter. New York and California have banned trans-fats in restaurant foods. Canada and Britain have considered it and countless food companies have dropped them as an ingredient.

Veronique Chajes of the French national scientific research center at the University of Paris-South and colleagues studied women taking part in a large European cancer trial.

They looked at blood samples collected between 1995 and 1998 from 25,000 women who had volunteered to report on their eating and lifestyle habits and then be followed for years to see if they developed cancer.

They studied 363 women diagnosed with breast cancer, comparing their blood levels of fatty acids with those of women without cancer.

The higher the levels of trans-fatty acids, the more likely a woman was to have cancer, Chajes and colleagues found.

Women with higher levels of omega-3 fatty acids, being studied for their potential benefits to health, were not any less likely to have breast cancer, the researchers found.

Obese women are more likely to develop breast cancer, among other types of cancer, and high-fat diets are also linked with breast cancer.

Trans-fats can be found in cooking fats, baked goods, snacks and a variety of other prepared foods. Omega-3 fatty acids are found in fatty fish such as salmon, walnuts and leafy green vegetables.