May 21, 1982

International Britain rejected an appeal from the United Nations for further negotiations on the Falkland Islands dispute, and the Cabinet was said to authorize the British battle fleet in the South Atlantic to open a new assault. In rejecting the appeal, Prime Minister Margaret Thatcher told the House of Commons that Argentina had in effect turned down Britain's final proposals for a settlement and that ''it is inconceivable that they would now come genuinely to accept'' the ideas proposed Wednesday night by United Nations Secretary General Javier Perez de Cuellar. (Page A1, Columns 3-6.) Argentina braced for a possible attack by Britain on the Falkland Islands, while Foreign Minister Nicanor Costa Mendez insisted that Argentina still considered the United Nations mediation effort to be alive. ''Argentina is engaged in the mission of the Secretary General, and it continues to insist upon his mission,'' the Foreign Minister said at a news conference, apparently before learning that Secretary General Javier Perez de Cuellar said tonight in New York that his peace efforts had failed. (A1:4-5.)

OPEC oil ministers expressed sharply different views today about whether to abandon their two-month-old program to limit output by the 13 member nations, now that falling prices seem to have stabilized. The disagreement marked the start of the semiannual meeting here of the Organization of Petroleum Exporting Countries. The session opened amid considerable pomp, as OPEC celebrated what it considers its success in managing production levels. ''It's very important for OPEC to move toward permanent controls,'' said Belkacem Nabi, Algeria's Minister of Energy and Petrochemical Industries, before the meetings.

A class-action suit was filed against the state yesterday on behalf of the estimated 6,000 homeless men and women who have been released from state psychiatric hospitals and now live on the streets of New York City. The suit seeks a court order directing the state to provide residential facilities for this group of homeless people. It was hailed by Mayor Koch and by City Council President Carol Bellamy.

The Argentine military braced today for a possible British assault on the Falkland Islands, while Foreign Minister Nicanor Costa Mendez insisted that Argentina still considered the United Nations mediation effort to be alive. ''Argentina is engaged in the mission of the Secretary General, and it continues to insist upon his mission,'' the Foreign Minister said at a news conference, apparently before he learned that Secretary General Javier Perez de Cuellar said tonight in New York that his peace efforts had failed. Asked about reports that Peru had presented a new peace plan, Mr. Costa Mendez said it had, but declined to give details. He said the plan ''lay in the realm of diplomatic confidentiality.'' A previous Peruvian plan, presented with the United States, was rejected by Argentina on May 3. On the same day Peru offered military aid to Argentina in its conflict with Britain, an offer that Peru renewed today.

Prime Minister Margaret Thatcher today rejected a United Nations appeal for further negotiations on the Falkland Islands dispute. The Cabinet was said to have authorized the British battle fleet in the South Atlantic to open a new offensive. Mrs. Thatcher told a rapt and expectant House of Commons that the Argentines had in effect turned down Britain's final proposals for a settlement of the seven-week-old crisis. She said that ''it is inconceivable that they would now come genuinely to accept'' new ideas that were proposed Wednesday night by Secretary General Javier Perez de Cuellar. U.N. Chief Concedes Failure At the United Nations, Mr. Perez de Cuellar conceded that his efforts to resolve the crisis had failed and said the conflict could now worsen. (Page A12.)