British Prime Minister Margaret Thatcher, bluntly recalling that British troops had shed blood in their attempt to regain the Falklands, told President Reagan today that it was important "not to turn our backs on them or on the Falklanders."

Thatcher's stand, which was confirmed both by U.S. and British sources, seemed to dash U.S. and French hopes of any last-minute diplomatic solution that would avert a battle to take the Falklands capital of Stanley.

U.S. Secretary of State Alexander M. Haig Jr. and British Foreign Secretary Francis Pym insisted that Reagan had not even requested a softening of the British position. Haig's comments contradicted a series of statements during the week by several administration officials who said the president hoped to persuade the prime minister to ease her insistence on a battlefield solution.

[In Washington, White House sources confirmed today that Reagan had intended before the meeting to suggest to Thatcher that Britain reach some accommodation with Argentina, staff writer Herbert H. Denton reported.]

Yesterday, a White House official said Reagan wanted Thatcher to display "Churchillian magnanimity" toward the Argentines. Today, a British source responded to this idea by saying: "We've been asked to be magnanimous. The question is how you define it."

The way the British are defining it, it seems apparent here, applies to conduct after Stanley is taken and the Argentines defeated, not before. And Reagan seems to have come over to the British camp on this issue. One view of a British source was that both the president and the prime minister shared a feeling "that they'd like to get the assault over as soon as possible."

It was not clear, however, whether Reagan had simply changed his mind about persuading Thatcher after telling aides that he would make the attempt, or whether he became convinced during the meeting that the British prime minister would not bend no matter what he said.

On Wednesday, the Los Angeles Times quoted administration officials as saying that Reagan had telephoned Thatcher and urged "an equitable solution" that would "result in minimum hurt to both sides."

Yesterday, a high-ranking White House official told The Washington Post that the United States wanted the British to stop a step short of victory in the Falklands war and allow time for a final attempt at a diplomatic solution. Administration officials were quoted the same day by United Press International as saying that Reagan would urge Thatcher to be "more magnanimous toward Argentina."

Today, the administration seemed to reverse course without describing what had happened to cause the apparent change in the president's position.

Confronted by Thatcher's determination to see the battle of Stanley through to victory, the president reverted to his earlier total support of the British position.

Haig dismissed the statements by administration officials during the week as "speculation" and said that Reagan had not suggested a "pause" in the fighting. "We would not presume to, and I think I want to get the record straight on that."

The secretary also said "both leaders are aware of the desire to keep bloodshed at a minimum."

Pym said Reagan reassured Thatcher of U.S. support for the British position. He said French President Francois Mitterrand made a similar reassurance in another private meeting with Thatcher, even though French officials the day before were expressing hope that Reagan could convince Thatcher to modify her stand.

Neither Haig nor Pym was present during the 90-minute private meeting between Reagan and Thatcher.

Haig, in his briefing, also brought up the issue of terrorism, which he described as "an international plague." He said the issue would be discussed by representatives of the seven nations who gathered for their economic summit meeting at Versailles tonight.

The leaders met for an informal dinner tonight, and a British delegation spokesman said most of the conversation centered on discussion of the Falklands. According to the spokesman, Thatcher "found them supportive and no one asked for a pause" in the British advance on the islands.

Haig denounced as "a cowardly terrorist attack" a pre-dawn bombing of The American School near Paris today, which inflicted no injuries. An anarchist group, Direct Action, claimed responsibility.

He did not characterize the shooting of an Israeli diplomat in London on yesterday but said he believed that today's bombing of Beirut probably was an Israeli retaliation for that action.

Reagan also touched upon terrorism when he visited the American Embassy here and paid tribute to Lt. Col. Charles R. Ray, the U.S. military attache who was shot and killed by terrorists in Paris on Jan. 18.

"From experience, I know it's no fun being a target no matter where on earth you're standing--even outside the Hilton Hotel," the president said in reference to the attempt on his own life last year in which he was wounded. "But the safety of our diplomats is of paramount concern to us. President Mitterrand has also put in motion steps to control the threat of terrorism. We're encouraging him . . . to continue this effort."