British Prime Minister Margaret Thatcher emerged from a White House meeting with President Reagan yesterday with a toughly worded reassertion of British sovereignty over the Falkland Islands and a disavowal of U.N. diplomatic efforts to negotiate a long-term solution for the islands.

"There is no question of sovereignty to discuss," Thatcher told reporters soon after she ended her fourth meeting with Reagan in the last three weeks.

Despite concern among some U.S. officials that the firm British stand, backed by the administration, is damaging U.S. relations with Latin American nations, there was no sign yesterday that the president had exerted any pressure on Thatcher to modify her views.

"We wouldn't expect anyone to tell us how to deal with our property, and we're not going to presume to tell her," Secretary of State Alexander M. Haig Jr. said in discussing the Reagan-Thatcher meeting.

Thatcher disavowed U.N. Resolution 502, which was passed April 3, the day after Argentina invaded the Falklands, and which sought to secure a diplomatic solution to the fighting.

The resolution called for cessation of hostilities, an Argentine withdrawal and a negotiated settlement on the Falklands' future.

But Thatcher took the position in her news conference yesterday that the resolution was not operative because the Argentines had not withdrawn but had been expelled from the islands by military force.

"When you have a three-legged stool, it can't stand on two legs," Thatcher said. She was in Washington for the afternoon after delivering a strongly worded anti-Soviet speech to the United Nations yesterday morning.

After Thatcher's news conference Haig was asked at a White House briefing for his view on Resolution 502. He replied, "The prime minister has stated it very clearly."

The question of a long-term solution for the Falklands did not come up in blandly worded statements issued by Reagan and Thatcher on the South Lawn of the White House after their meeting.

When a reporter shouted a question at Reagan after the meeting about whether the United States supports the U.N. resolution, the president at first declined to answer on grounds that the departure session was a "photo opportunity," not a time for answering questions.

Then he said, "I think we've made our position plain and clear in what we've tried to do . . . . "

Thatcher briskly cut in on him, saying, "We've very grateful to the president for everything they have done to help."

Reagan and Thatcher met three times--in Versailles, London and Bonn--during the president's recent trip to Europe. After each meeting Thatcher promptly came forward to face questions from the press, while Reagan declined to meet with reporters, a practice he followed again yesterday.

In contrast to the polite departure statements of both leaders, Thatcher gave a bristling performance at the subsequent news conference, saying repeatedly that the Falklands are British and will remain so.

She observed that only 30 Argentines were on the islands before the Argentine invasion, that most of the Falklanders are of British stock and that some had lived there for as long as seven generations.

The only way in which Britain would deal with the question of sovereignty, Thatcher said, would be to discuss self-government at a future date if that is what the islands' residents desire.

Judging from the remarks by Thatcher and Haig, Reagan again did very little, if anything, to attempt to soften Thatcher's consistent hard line on the Falklands.

The two leaders apparently did not discuss whether U.S. sanctions imposed by the Reagan administration against Argentina should continue.

Thatcher said she "rather understood that the United States intends to continue with the sanctions." Haig said the question of sanctions is "under review," but cautioned against interpreting this as a sign that any immediate decision will be made to lift them.

Haig indicated that the Anglo-American stance toward Argentina will not change until there is a cessation of the conflict and until a new government consolidates its authority in Argentina.

The only apparent difference between Thatcher and Reagan at yesterday's meeting was over the president's action of last Friday in extending sanctions against the Soviet Union in retaliation for continuation of martial law in Poland. Thatcher said these sanctions were hurting a British firm. Reagan replied that he understood they are a burden, but that he believed they are also necessary.

"Just as Mrs. Thatcher has her principles, our president has his," Haig said in explaining Reagan's stand.

Earlier, in a speech to the U.N. disarmament conference, Thatcher expressed principles about the Soviet Union that she shares with Reagan, questioning intentions of the "Soviet empire" and the advantages she said the communist bloc enjoys in conventional military forces.

Echoing views stated by Reagan at the United Nations last week, Thatcher said events in Poland and Afghanistan demonstrated an instability within the Soviet bloc.

"Thus, the need to secure a better balance in conventional arms becomes even more imperative," she said.

U.S. representatives in the General Assembly applauded enthusiastically at the end of her 20-minute address, but the Soviet delegation remained quiet. Delegations from Argentina, Guatemala and Bolivia boycotted the speech.