A senior United Nations official predicted Washington would ignore a World Court ruling that said the United States violated a U.N. treaty by trying to close the PLO's New York office. In an advisory opinion issued Tuesday, the International Court of Justice, as the court is formally known, ordered the United States to submit to arbitration in its dispute with the United Nations over the Palestine Liberation Organization's observer mission to the United Nations. The court is the judicial arm of the United Nations. It has no enforcement powers and depends on voluntary adherence to its rulings. The Reagan administration's dispute with the U.N. arose from legislation signed into law by the U.S. Congress in December that classifies the PLO as a ``terrorist organization'' and bars it from operating on American soil. The U.S. government maintains that the anti-terrorist law takes precedence over any international agreements. Carl-August Fleischhauer of West Germany, the U.N. undersecretary-general for legal affairs, told reporters after Tuesday's ruling that he doubted the United States would submit to arbitration. ``We want harmonious relations with our host country,'' Fleischhauer said. ``We are ready to go to arbitration. It is now basically for the United States government to decide what conclusions it draws from this ruling.'' ``It is a pleasure to be proved right,'' Fleischhauer's boss, U.N. Secretary-General Javier Perez de Cuellar, said of the ruling during a press conference in New York. Although the court took pains to dodge the question of whether Washington's attempt to close the PLO office is legal, the ruling was seen in Vienna as an implicit condemnation of the U.S. effort. The court said the U.S. Justice Department's shutdown order violated the Headquarters Agreement by not submitting the issue to arbitration first. That 1947 pact between the Truman Administration and the U.N. guarantees diplomats unimpeded access to U.N. headquarters in New York. ``The United States is bound to respect the obligation to have recourse to arbitration,'' said the court's 15 judges. The Headquarters Agreement mandates independent arbitration of any disputes arising from it, but the United States has rejected any such arbitration. The opinion marked the second time in two years the court has ruled against the United States. In the previous case, brought by Nicaragua, the United States boycotted the proceedings and ignored the court's order to halt its support for rebels trying to overthrow the Managua government. The PLO was granted permanent observer status by the U.N. General Assembly in 1974, and its U.N. mission has been operating ever since out of an office in Manhattan. Its information office in Washington, D.C., was closed prior to the passage of the anti-terrorist legislation. Early this year, the U.S. government said arbitration of the PLO mission issue was premature, and noted that the anti-terrorist law would only come into force on March 22. When the PLO refused to comply with U.S. Attorney General Edwin Meese III's March 22 order to close its New York operation, the U.S. government filed suit in a New York federal court. That lawsuit is pending. Washington's attempt to close the PLO office has been condemned by every U.N. member nation except Israel.