The European Union threatened to slap record sanctions on U.S. products, saying a new U.S. export tax system is just as unpalatable as the one struck down earlier this year by the World Trade Organization. If enforced, the sanctions would set the two economic giants on their biggest trade dispute yet. The 15-nation European Union asked the WTO to review the law President Clinton signed Thursday that institutes the new system. The EU said it would impose $4 billion in sanctions on everything from steel to corn flakes if the WTO rules that the new tax system is illegal. The new law was designed to address WTO objections by creating new tax breaks that would apply equally to U.S. exports and to products the companies manufacture in their overseas plants.