Ireland called today for an urgent meeting of the United Nations Security Council as Britain's European Community partners abandoned their previously united backing for London in the Falkland Islands crisis.

Joining the Irish proposal for a cease-fire and U.N. involvement in negotiations between Argentina and Britain were the Italian and West German governments.

In addition, the Irish announced that they will seek the lifting of economic sanctions taken by the 10-nation community on April 10 to back Britain's diplomatic efforts to force Argentina to withdraw its occupation troops from the Falklands.

U.N. Secretary General Javier Perez de Cuellar said Tuesday that he has set a deadline of midday Wednesday for Britain and Argentina to reply to proposals for resolving the crisis, and a senior official said the Security Council will be convened if the reaction is negative, special correspondent Michael J. Berlin reported from the United Nations.

Triggering the shift by Britain's major European allies was the ferocity of the hostilities in the South Atlantic, especially the feared high death toll aboard the Argentine cruiser General Belgrano torpedoed and sunk by a British submarine, although the day's developments came before the British announced that one of their destroyers had been sunk by an Argentine missile.

Gone was the mood of self-congratulation generated within days of Argentina's seizure of the British-ruled islands when London's European partners unanimously applied economic sanctions against the Buenos Aires government last month.

As that unity evaporated today, Irish Defense Minister Patrick Power told reporters in Dublin before a Cabinet session, "We felt Argentina was the first aggressor and originally backed Britain, but obviously the British themselves are very much the aggressors now, and we are taking a neutral stance."

Echoing a line developed by West German, Italian and Danish leaders, the Irish government said it was "appalled by the outbreak of what amounts to open war."

In Bonn, West German Chancellor Helmut Schmidt said "we are concerned" about the hostilities. Later, government spokesman Lothar Ruehl expressed Bonn's "surprise and dismay" at the sinking of the cruiser, "in which many persons obviously lost their lives."

He appealed for a cease-fire and asked both sides to "explore all possibilities for a peaceful solution" of the crisis.

France, previously the most solid supporter of the British stand on the islands, expressed its "consternation" at the cruiser's sinking in a Foreign Ministry spokesman's statement.

Andre Chandernagor, deputy foreign minister for European affairs, told a radio station that hostilities should "end as quickly as possible" and regretted that the crisis had "degenerated into war."

Italian Foreign Minister Emilio Colombo told a congress of his Christian Democratic Party that U.N. Secretary General Javier Perez de Cuellar should intervene in the conflict and said the "first step" should be a cease-fire.

Betraying Italy's misgivings, he said, "Britain is a member of the European Community to which we belong, and Argentina is . . . linked with Italy in culture and blood." Many Argentines are of Italian stock.

In Brussels, European Community analysts have been predicting since the weekend's hostilities that Britain probably would have to cease military operations and agree to a U.N.-sponsored peace effort if it hoped to have the ban on Argentine imports renewed after they expire May 17.

Top civil servants met privately outside Brussels to discuss the Falklands crisis before a foreign ministers' gathering this weekend on that and other problems.

For several days Denmark, Ireland, Italy and West Germany argued that Britain's use of force was not authorized in the original resolutions imposing the sanctions.

The sanctions were designed to lend support for Secretary of State Alexander M. Haig Jr.'s unsuccessful peace mission.

Even France had made it clear from the start that its support was limited to demands that Argentina withdraw from the Falklands and in no way implied backing for British claims to sovereignty.