"We urge you to take that step without delay," the senators, led by incoming Senate Finance Committee Chairman Charles Grassley, wrote.

For the past four years, the EU has enforced a moratorium on the approval of new biotech products, ranging from agricultural goods to pharmaceuticals. The ban stems from widespread consumer fears in Europe over the safety of biotech foods. The United States says such products present no danger to health.

The United States is a major producer of biotech foods, with about 70 percent of soybeans and 25 percent of corn grown from genetically modified seeds. The biotech company Monsanto hopes to bring to market biotech wheat.

The WTO has ruled illegal the EU's moratorium on new biotech products. While the European Commission has been trying to lift that ban, it is meeting resistance from some member countries.

U.S. farm and agriculture groups have also urged the Bush administration to file a formal complaint with the WTO.

The senators' letter said formal action was needed because there was no sign the EU would voluntarily open its market. The moratorium costs the United States $300 million annually in lost corn sales, it said.

Bush's Cabinet is expected to debate whether a WTO complaint should be filed.

U.S. officials say a formal complaint could create problems, such as possible EU retaliation in other trade disputes and a strain in ties with Europe at a time when Washington is seeking to build a coalition against Iraq.

A U.S. agriculture industry source last week told Reuters the letter from the senators was being crafted in part to give the White House additional political cover for launching a trade case with the WTO, based in Geneva.