Prime Minister Margaret Thatcher said tonight that it has become impossible to negotiate a settlement of the Falkland Islands dispute, and government sources said British forces have been ordered to begin landings and hit-and-run raids.

"Difficult days lie ahead," Thatcher told the House of Commons after detailing Britain's version of a breakdown in negotiations with Argentina through the United Nations. "The gravity of the situation will be apparent to the House and the nation."

At the United Nations, Secretary General Javier Perez de Cuellar informed the Security Council that his mediation effort had ended in failure. Story, Page A23.

A British Defense Ministry source said that with no hope left for a negotiated settlement, military action will be escalated "quite steeply and quite quickly." He said timing and tactics have been left to the task force commander, Rear Adm. Sandy Woodward, and may be dictated in part by weather in the South Atlantic.

Senior government sources said they expected widespread British landings and hit-and-run attacks to begin by the weekend at the latest. Rather than a concentrated "D-Day type" landing or frontal assault on Argentine forces around the islands' capital of Stanley, government sources said there would be coordinated, escalating attacks to cut off and disable concentrations of Argentine troops and force them into surrender with a minimum of casualties.

"The name of the game now is attrition," a Defense Ministry source said.

In Washington, the Special Situation Group of top diplomatic, defense and intelligence officials, set up under the chairmanship of Vice President Bush to deal with crisis situations, met to discuss the Falklands situation for the first time. Administration sources said, however, that the meeting was only to review and monitor the situation. They said there were no plans for the United States to attempt to play a new mediating role.

These sources also said the United States is not giving Britain any assistance beyond the fuel and intelligence data it has been providing since the early days of the crisis. Administration sources quoted by United Press International said the United States has provided U.S. Air Force KC135 tanker aircraft to take over British refueling functions in Europe to enable similar British planes to fly missions in connection with the Falklands crisis.

Meanwhile, U.S. weather satellite photos taken Thursday morning showed low-lying clouds as well as rain or snow over the Falklands--weather suitable for Britain's low-flying helicopters but poor for Argentina's high-altitude fighter-bombers. The pattern indicated winds of 30 knots or more, which would cause rough seas.

In by far her strongest verbal attack yet on the military government of Argentina, Thatcher said, "We have reached this very serious situation because the Argentine clearly decided at the outset of negotiations that they would cling to the spoils of invasion and occupation by thwarting at every turn all the attempts that have been made to solve the conflict by peaceful means."

Since April 2, when Argentina seized the Falklands, which had been under disputed British rule for 149 years, Thatcher said, Argentina has "responded to the efforts to find a negotiated solution with obduracy and delay, deception and bad faith."

Thatcher said Argentina yesterday rejected Britain's final peace proposals, which represented "the limit to which the government believed it was right to go" in making "reasonable practical changes" in the British negotiating position without compromising principle. Because "these proposals have been rejected," she told Parliament, "they are no longer on the table."

At the same time, Thatcher said, "it was manifestly impossible" for Britain to accept Argentina's final counterproposal. She said that "it retracted virtually all the movement their representative had shown" during the negotiations conducted by U.N. Secretary General Javier Perez de Cuellar.

Thatcher argued that Argentina "plainly calculated to put us at an enormous disadvantage" by demanding that British forces return to their bases rather than withdrawing 150 miles from the Falklands, as the British proposed for both sides. She strongly objected to an Argentine demand that Britain also withdraw from the island of South Georgia, which Thatcher said is a legally separate British territory 800 miles from the Falklands.

Thatcher said Argentina's proposal for administration of the islands after mutual withdrawal would "change the nature of Falklands society and so prejudge their future" by rejecting any role for the executive and legislative councils of the islands' 1,800 inhabitants of British heritage and then "flooding" the Falklands with Argentine nationals.

Thatcher also said it was "inconceivable" that negotiations could be revived on the basis of suggestions for compromise that Perez de Cuellar sent to both the British and Argentine governments late last night. She said some of the suggestions "differ fundamentally from the present Argentine position" and others "differ in certain important respects" from Britain's latest position, "which we described as the furthest we could go."

Even if Perez de Cuellar's suggestions "were acceptable to both parties as a basis for negotiation," Thatcher said, "that negotiation would take many days if not weeks to reach either success or failure. We have been through this often before and each time have been met with Argentine obduracy and procrastination."

"Argentina has rejected proposal after proposal," Thatcher told Parliament. "One is bound to ask whether the junta have ever intended to seek a peaceful settlement or whether they have sought merely to confuse and prolong the negotiations while remaining in illegal possession of the islands.

"I believe that if we had a dozen more negotiations the tactics and results would be the same."

Thatcher told Perez de Cuellar through Britain's ambassador to the United Nations that her government could not negotiate on the basis of his suggestions unless it was clear that Argentina "would now come genuinely to accept those of the secretary general's ideas that closely resemble our own."

"At the same time, we are reminding him," she added, "the negotiations do not close any military options."

Task force commander Woodward now has the authority to exercise a wide range of military options to recapture the Falklands, Defense Ministry sources said, with only attacks on the Argentine mainland ruled out for now. The sources said landings and raids would be made at many locations, in an effort to harass isolated Argentine troops and panic them despite their superior numbers.

British forces landing on the Falklands would defend themselves against mainland-based Argentine warplanes with mobile surface-to-air Rapier missiles and Harrier jets that can land and take off vertically from any flat ground. Light armored vehicles and tanks could be landed from large landing craft. Commandos would be landed by helicopter.

Military analysts here expect the landings to be carried out in darkness, so British ships could retreat by daylight beyond the range of Argentine planes from the mainland.

The British forces have continued to attack the Argentine defenders with daily naval bombardment of positions around Stanley and its airport and periodic Harrier bombing raids. The British Defense Ministry today confirmed a bombing attack late yesterday by two carrier-based Sea Harriers on "military targets eight miles from Stanley."

Britain's shift to these military tactics from an emphasis on the U.N. talks was supported by politicians of all parties in the House of Commons despite vocal opposition from a group of Labor members led by left-winger Tony Benn, who mustered 33 votes against the government at the end of tonight's debate.

Labor leader Michael Foot said the government's proposals for a settlement were "fair" and "if escalation of force is necessary, let it be as swift and successful as possible."