In an effort to break the deadlock over the Falklands dispute, Britain has dropped its demand that Argentina explicitly guarantee that there will be no further hostilities in the contested South Atlantic region.

Instead, in a letter addressed to the United Nations Security Council, the British government said merely that it "looks forward to receiving positive indications which will allow it to conclude" that the conflict is over as far as Argentina is concerned.

What that means, officials said, is that Argentina can bring the Falklands saga to a speedy close simply by signaling in whatever way it chooses that it will not reopen hostilities--in effect, maintaining a cease-fire without admitting it is doing so.

Last week, Argentina conveyed to Britain through a private channel a willingness to accept prisoners being delivered to its shores on British ships. That is the kind of breakthrough officials here are seeking again. Until now Britain had insisted that Argentina publicly promise that it would not again attack British forces. In return, Britain promised to drop economic sanctions against Argentina, lift restrictions on Argentine shipping in the region and return about 1,000 Argentine officers and military specialists still being held prisoner.

Britain's willingness to show flexibility on the crucial question of an Argentine pledge to end hostilities reflects a sense that political turmoil in Argentina probably means there is no real prospect of an explicit statement of a cease-fire.

"No one who might make it has surfaced in the struggles over leadership of the junta," one official said.

Moreover, Prime Minister Margaret Thatcher is about to lose her free hand in managing the Falklands crisis. She returned this morning from her one-day visit to the United Nations and Washington to hear sharp criticism from political opponents who seized upon a letter she wrote shortly before the Argentine invasion asserting that the islands were sufficiently protected.

"This letter reveals the stark negligence on the part of Mrs. Thatcher and her government," a Labor Party spokesman said, "which led directly to the fiasco of April 2, the day of the invasion."

Critics hope to hold Thatcher responsible for Britain's failure to foresee Argentina's seizure of the Falklands and thereby diminish the political glory she has achieved from the subsequent British military successes.

The immediate objective of Thatcher's opponents is to control the scope of the inquiry into failures in intelligence and strategy that forced Britain into a costly war to retake the islands. Thatcher wants the review to encompass a 20-year period of British negotiations with Argentina. The opposition Labor, Social Democratic and Liberal parties are pressing for what they call a "short, sharp" review of government actions in the period just before the invasion. This, they believe, will prove embarrassing to Thatcher.

In this return to politics as usual, Thatcher may well find that maintaining an armada in the South Atlantic and 9,000 to 10,000 troops on the Falkland Islands is an increasingly heavy burden. Therefore, the prime minister is starting to look for ways to lessen Britain's military requirements in that region.