Prime Minister Margaret Thatcher warned Argentina today that time for negotiating a peace settlement is "getting extremely short" before Britain follows its recapture of South Georgia with military action against Argentine forces on the Falklands.

Amid growing speculation among politicians and analysts here that Britain's next military move may be made within 48 hours, Thatcher refused to discuss specific options or timing. But she noted that the British fleet is getting "closer to the Falklands" and told Parliament, "It's obvious you simply cannot go on sitting there forever with a very large task force."

Thatcher's warning came during a report to Parliament on the surrender today of the last defenders of South Georgia to British forces.

In Argentina, the military government maintained a studied silence over Britain's recapture of the island of South Georgia while the country reacted with a mixture of confusion and renewed patriotism to the limited news of the military setback. Details on Page A15

After a little more than a day of sporadic, light fighting, Thatcher told Parliament, the commander of the Argentine forces on South Georgia "formally surrendered" this morning. Military prisoners included 40 who gave up at the Argentine garrison at the port of Grytviken yesterday afternoon, 100 crewmen and garrison reinforcements aboard the Argentine submarine, which was attacked and disabled by British combat helicopters, yesterday morning, and 16 who surrendered at Leith, north of Grytviken, this morning.

Reporting that the only casualty was an injured Argentine sailor on the submarine, Thatcher said, "British forces throughout the operation used the minimum force necessary to achieve a successful outcome."

Negotiations "will have to come to a head soon," Thatcher said later in her first national television appearance in three weeks. Discussing details of the negotiations for the first time in an interview, she said she would not yield on three points of principle: complete Argentine withdrawal from the Falklands; restoration at least temporarily of British administration, perhaps with an international peace-keeping force; and guaranteed self-determination for the islands' 1,800 English-speaking inhabitants in any future sovereignty talks.

"You cannot just go on indefinitely with negotiations," Thatcher warned repeatedly. "In the meantime, it will get more and more difficult for us to use the military option 8,000 miles from home at the onset of winter in very terrible weather, gales and freezing."

Appearing to rule out an extended blockade of the Falklands with British warships already in nearby waters, Thatcher said, "We have two carriers; they're there. We have a large number of ships; they're there. We can replace some of them, but not all of them.

"You have to decide what is the best time for the military option," she said, "bearing in mind the safety of your own people and their capacity to do whatever they are to do with a minimum loss of life."

Thatcher told lawmakers she decided to recapture South Georgia with a much smaller number of ships and men for the same reason. "They the Argentines have had three weeks in which to start to withdraw their forces and negotiate" through U.S. Secretary of State Alexander M. Haig Jr., she said. "We had to retake South Georgia at the best possible time."

Thatcher also said on television tonight, "I want minimum escalation" of the conflict. But she refused to rule out military options and said she hoped the recapture of South Georgia would make Argentina realize that "we are quietly determined in pursuit of a principle."

Thatcher remained unyielding about the British bargaining position in the negotiations being conducted by Haig. She said in the TV interview that all Argentine forces would have to be withdrawn from the Falklands before the British task force would be pulled back, complete British civil administration would have to be restored, and self-determination would have to be maintained for the 1,800 British residents in negotiations about the Falklands' long-term sovereignty.

Referring to the islanders as "our people," Thatcher said, "The sticking point for us is their right to self-determination. If they wish to remain British, we must stand by them."

She said she would not allow the Argentine flag to fly on the Falklands "as a sign of sovereignty in any way," immediately after a military withdrawal, "although it could fly over an Argentine mission.

"British law and British executive and legislative councils" also would have to be returned at least temporarily after an Argentine withdrawal, Thatcher said. But she suggested an international group such as the Sinai peace-keeping force could be introduced at the same time "to supervise the withdrawal."

Thatcher said Haig has been considering a force led by the United States or the United Nations, but she indicated her own doubts about a U.N. force.

A number of members of Parliament urged that all opportunities for a diplomatic settlement be exhausted before the military conflict is intensified. But others in Thatcher's Conservative Party pressed her to move quickly on the diplomatic and military fronts to take advantage of the easy British recapture of lightly defended South Georgia.

Refusing to say how her government might try to speed up the negotiations or how a settlement could be achieved, Thatcher said only, "We remain in close touch with Mr. Haig."

David Owen, a former foreign secretary, who is now foreign affairs spokesman for the Social Democratic Party, said, "It is right to give Secretary Haig a few more days." But he added, "The time is approaching" when, if there is no progress in the negotiations, "the United States will have to make the decision to apply economic sanctions" against Argentina.

He and opposition Labor Party leader Michael Foot, however, also pressed Thatcher on what more she is doing to seek a peaceful settlement and whether she would ask for help from the United Nations if Haig's mission fails. Saying "the search for peace should never be torpedoed by us," Foot asked Thatcher, "how are we to be assured there will be no dangerous escalation of the crisis?"

Tony Benn, leader of a group of left-wing Labor members of Parliament that has been calling for the British task force to be turned around while negotiations continue, warned that "if the prime minister continues to underestimate the importance of negotiations and proceeds with the war, the responsibility for loss of life will rest on her shoulders."

But opposition members of Parliament also joined Conservatives in praising British forces for retaking South Georgia without loss of life.