With $5 billion on the line, the American business community has quietly launched an all-out effort to preserve a lucrative tax exemption.

Last month, a World Trade Organization panel ruled against a U.S. program in which some 7,000 companies enjoy tax breaks by setting up offshore foreign sales corporations, or FSCs. The WTO, agreeing with a complaint filed by the European Union, held that the program creates illegal subsidies and gave the United States until Oct. 1 to make changes or suffer retaliation.