Secretary General Javier Perez de Cuellar said today he had set a deadline of midday Wednesday for Britain and Argentina to reply to his proposals for resolving the Falkland Islands crisis.

He asked for an urgent response, a senior U.N. official said, in the belief that once British forces have staged a landing to recapture the islands, a settlement would be even more difficult to negotiate.

If the reaction is negative, the official said, the Security Council will be convened in public session in response to a request from Ireland--something that neither the British nor the Argentines desire for now.

Irish Prime Minister Charles Haughey, who instructed his U.N. ambassador today to ask for the council meeting, agreed to the secretary general's request to delay the debate until the replies are in.

Informal consultations on the request were scheduled for late Wednesday afternoon.

If the responses are positive, Ireland is expected to agree to a further delay, U.N. officials said.

The U.N. proposals, presented to both sides by Perez de Cuellar on Sunday, has not been revealed publicly, but diplomats said it includes a simultaneous, phased withdrawal by the British fleet and Argentine troops on the Falklands; negotiations on ultimate sovereignty over the islands under a U.N. representative, and a U.N. administration on the Falklands while the talks continue.

Both sides have expressed "cautious interest" in the proposals, informed officials said. But they could not predict whether the casualties suffered by both Britain and Argentina since the offer was made would increase their willingness to come to terms--or stiffen their will to fight.