Prime Minister Margaret Thatcher is prepared to declare Thursday that negotiations with Argentina have failed and Britain is taking military action to retake the Falkland Islands, well-informed diplomatic and political sources said here tonight.

Thatcher is expected to detail, in an emergency parliamentary debate, what she called today the "big gap" between the final negotiating positions presented by Britain and Argentina to U.N. Secretary General Javier Perez de Cuellar in New York this week to support her contention that Argentina is to blame for the breakdown in negotiations.

Perez de Cuellar said tonight that he had appealed to Thatcher and Argentine President Leopoldo Galtieri by telephone today to consider his own last-minute proposals to bridge their differences, special correspondent Michael J. Berlin reported. The secretary general told the Security Council in a closed consultation that Galtieri had responded positively but Thatcher had been "ambiguous." He said, however, that the fact that she had not rejected his effort, the details of which were not made public, convinced him to pursue this last chance.

While he insisted in a later statement to the press that an agreement still could be reached "without prejudice to the rights, claims or position of either party," the secretary general acknowledged, "We are at a very dangerous point" where "the time left for negotiation must be measured in hours."

The negotiations had reached an impasse earlier in the day when the British ambassador, Sir Anthony Parsons, reported to the secretary general that the Argentine reply to proposals made by London Monday were unacceptable. The Security Council then granted Perez de Cuellar's request for two more days to pursue his own peace plan, but Parsons said he was "very pessimistic" that agreement could be reached.

Many politicians and diplomats here also expect Thatcher to announce that British forces have begun landing on the Falklands, although some government sources cautioned that "this would not necessarily follow immediately after a breakdown."

Military analysts here expected possibly widespread landings by British troops on both West and East Falkland islands. But they were uncertain whether there would be an immediate direct assault on the islands' capital of Stanley, which is defended by the largest concentration of Argentine occupation forces. Instead, they foresaw an attempt to close in gradually around Stanley and persuade its Argentine garrison to surrender.

Britain's Press Association news agency reported tonight that many of the 4,000 to 5,000 Marines and paratroopers with Britain's task force around the Falklands have been transferred to assault ships. British correspondents with the task force yesterday described other final preparations for an invasion, including the transfer of additional Harrier jump jets from a transport ship to the aircraft carriers Hermes and Invincible.

A British Broadcasting Corp. reporter on the Hermes said a British destroyer bombarded targets south of Stanley today while Harrier jets attacked military positions elsewhere on East Falkland.

A Defense Ministry spokesman today said only that "there has been no operational contact. The task force has been maintaining its patrols to enforce" Britain's blockade of the Falklands. Asked about such reported recent activities as new ship movements, increased antisubmarine patrols by Sea King helicopters and stepped-up Harrier reconnaissance flights over the Falklands, the spokesman said they were "not inconsistent with that function."

In one of yesterday's censored reports from the task force, a British correspondent aboard the Invincible said, "Tomorrow or soon after we could be in battle."

Defense Secretary John Nott indicated to British correspondents at a background briefing last night that only the timing of an invasion remained to be decided. He suggested that more raids might be made on key outlying military targets before an attempted recapture of Stanley.

But Nott also discussed the advantages of a rapid invasion if it could be accomplished with a minimal number of casualties. He said Argentine defenders had put up little resistance to the lightning hit-and-run raid last weekend by British commandos on a strategic airstrip on Pebble Island guarding West Falkland from the north.

Nott also repeated, according to accounts of British correspondents, that the task force "cannot go on tossing around forever in the South Atlantic and expect its equipment to be in peak condition for a landing."

The Defense Ministry announced that it will start tonight or Thursday to broadcast in Spanish to the estimated 9,000 Argentine troops on the Falklands from a British Broadcasting Corp. radio transmitter on Ascension Island.

By giving the Argentine forces what a Defense Ministry spokesman called "a better understanding" of military developments "than is currently available through wildly inaccurate Argentine reports," the British hope to convince them to surrender without a fight.

The reported precision of last weekend's raid on the Pebble Island airstrip made it appear certain that specially trained British commandos landed secretly on the Falklands some time ago and have been radioing to the task force information about locations of Argentine troops and likely landing sites. The Defense Ministry has admitted that two helicopters crash-landed while putting such commandos on the island of South Georgia, 800 miles east of the Falklands, three days before British forces recaptured it on April 25.

Commandos already on the Falklands are likely also to have contacted some of the 1,800 British subjects scattered around the islands. Many residents are believed to have left homes in Stanley to live in the many small settlements and shepherds' cottages unreachable by road in the countryside.

Naval officers directing the pinpoint bombardment of the Pebble Island airstrip by a British destroyer during last weekend's raid used a detailed map delineating a nearby settlement and kept shell fire away from it, according to British correspondents aboard the destroyer.

Between 5,800 and 6,000 Argentine troops are reported to be concentrated around Stanley, with as many as 2,000 more around Darwin, 55 miles west on East Falkland. Another 1,500 are believed to be at Fox Bay, Pebble Island and other locations on West Falkland. This leaves many suitable landing sites undefended but to recapture the Falklands, British troops must retake the Argentine positions.

The Argentines' armored vehicles and light tanks will be severely limited in utility by the lack of roads and the difficulty of the terrain outside the area around Stanley. Many Argentine planes on the Falkands also are believed to have been destroyed by British raids. But the task force's ships and the invading British troops will be seriously threatened by warplanes flying from the Argentine mainland.

British officials said they would continue after a landing to seek a negotiated settlement with Argentina on the Falklands' long-term future. But Foreign Secretary Francis Pym told a closed meeting of Conservative members of Parliament last night that "all bets would be off" concerning British terms in negotiations if the islands were recaptured.

In a BBC radio interview today, Thatcher said that transferring sovereignty to Argentina while maintaining British administration during a long lease period--suggested a year ago by her own government--would not be acceptable because "the islanders didn't want it."

London stock market prices and the value of the British pound sterling have fallen this week as chances of a peaceful settlement of the Falklands crisis ebbed. The exchange value of the pound fell back below $1.80 today and the 30-share Financial Times index of the London stock market dropped 10.5 points to 561.9, producing a loss of 29 points in the past four days.

British officials said today that the Thatcher government has taken new steps to try to encourage Pope John Paul II not to cancel his pastoral visit to Britain beginning May 28 because of the Falklands conflict. The British envoy to the Vatican yesterday transmitted an assurance from Thatcher to avoid all government contact with the pontiff here if that made it easier for him to come.

The Vatican announced tonight that the pope would conduct a solemn mass Friday or Saturday to pray for peace in the Falklands. Argentine and British clerics, including the two British archbishops now in Rome to plead with the pope to fulfill his visit here, have been invited to the mass.

Berlin also reported from the United Nations:

The council members met in private tonight to hear the secretary general's report on his mediation effort, after the final British proposal was delivered to Perez de Cuellar and transmitted to the Argentine negotiator, Enrique Ros.

The initial Argentine reply to the British proposals delivered on Monday was clearly unsatisfactory. One diplomat close to the British who saw the text called it "just a rag bag of restatements of known Argentine positions and requests for clarifications of issues they already know inside out."

Parsons told the Security Council and the press that the Argentine reply had left "very wide gaps" between the two sides.

The Argentines insist that their response delivered late last night contained substantive modifications of previous positions designed to bridge the gap. The Argentines, however, confirmed that they could not and would not go all the way to satisfy British demands.

Both sides confirm that a major difference between them is the British demand that the Falkland Islanders' legislative council continue to function during the interim U.N. administration of the islands. Argentina opposes such a formal role on the ground that it would weight the concurrent negotiations on ultimate sovereignty in Britain's favor.

Another sticking point is the British insistence on holding onto South Georgia and the Sandwich Islands while the talks go on, while Argentina insists that they, too, be put under U.N. administration and that the British troops there withdraw.

Perez de Cuellar told reporters that he had spoken to Galtieri and Thatcher "to express my views and my very great concern. The cost of failure in terms of human life and suffering is too high to permit us to give up our efforts."

He added that he had "suggested some ideas which I believe might be of assistance in overcoming the remaining points of difference." He did not elaborate.

Nonetheless, it became clear today that since Monday Perez de Cuellar's own input in the negotiating process has been minimal. He delivered the British text verbatim to Argentina, and today handed over the Argentine reply in both Spanish and English to the British--a departure from his previous practice of summarizing positions and adding his own suggestions. There is no indication that he has as yet proposed a formula of his own to resolve the differences that remain.