LONDON, JULY 31 -- Britain today turned down a U.S. request to send mine sweepers to the Persian Gulf, saying that such a step would contribute to an escalation of tension there.

Officials were anxious to emphasize that the decision, which they indicated had been made by Prime Minister Margaret Thatcher, implied no criticism of U.S. policy in the gulf.

The U.S. government declined any comment on the decision.

Shortly before her trip to Washington earlier this month, Thatcher had praised President Reagan's "clear and decisive leadership" over the gulf. She said she believed that "what the president has done is right," referring to the U.S. policy of providing naval escorts in the gulf for reflagged Kuwaiti tankers.

Officials today, however, made clear that it was not a policy in which Britain wanted to participate. "We are entitled to ask ourselves whether what we do will help or hinder" efforts to clear the gulf of mines, believed to have been laid by Iran, and to protect free navigation, Foreign Office Minister David Mellors said.

A Foreign Office statement said that Britain would continue "seeking by all possible means to persuade both Iran and Iraq" to respect a United Nations cease-fire call and refrain from hostile acts against shipping. "In this way," the statement said, "the underlying causes of tension in the gulf will be removed."

"We are keeping the situation . . . under constant review," it said, "but have no plans to send mine sweepers to the gulf in the present circumstances."

In Washington, French Defense Minister Andre Giraud said following two days of meetings with senior administration officials that France is studying "the possibility of contributing to a certain extent" toward improved security in the Persian Gulf but that the French "are not modifying the mission of our ships." Giraud said there are no mine sweepers in a French naval task force that is nearing the Persian Gulf and none in nearby ports that would be ready for action.

Giraud would answer only a few questions concerning the gulf situation. He gave no indication of any immediate changes in French policy that would aid the current U.S. effort in the gulf, where U.S. Navy ships are escorting U.S.-flagged Kuwaiti tankers through the waterway.

Britain's announcement today was the latest in a series of rejections of U.S. requests for help in the gulf. The requests became more urgent after one of the tankers, the Bridgeton, was damaged by a mine on July 24. Despite the unrepaired damage to the Bridgeton, the 1,200-foot supertanker began loading oil today for a return trip to the mouth of the Persian Gulf, under U.S. Navy escort, and could be ready to sail by this weekend.

Saudi Arabia has declined to lend its four mine sweepers to the effort. West Germany, which has 57 mine sweepers, said on Wednesday that its constitution bars the country from using its armed forces for anything but its own defense. Chancellor Helmut Kohl said West German vessels have "no business in the gulf."

Today, the Netherlands, with 20 mine sweepers, turned down an American request and said that Dutch forces would be sent to the gulf only as part of a U.N. peace-keeping force.

The feeling is widespread in Europe that administration policy implies a potentially open-ended military commitment that holds little promise of reducing tensions in the Iran-Iraq war and could increase the risk of terrorism abroad.

Britain's rejection of a U.S. request, made here yesterday by U.S. Ambassador Charles H. Price II, is likely to be seen as the most significant. Although U.S. officials were well aware that Britain's foreign and defense ministries were opposed to allowing any of the Royal Navy's 40 antimine vessels to travel to the gulf, they had hoped that the prime minister would overrule them.

Thatcher shares Reagan's belief in a strong defense and has been his most consistent supporter among the NATO allies. She allowed U.S. planes to take off from British airfields for last year's bombing raid against Libya and stood virtually alone in NATO in supporting the U.S. action.

According to officials here, Thatcher's backing for the increased U.S. presence in the gulf stems as much from her desire to support Reagan during a difficult period at home as from belief that the gulf policy will achieve its stated goals of stemming Iranian attacks.

For the past seven years, Britain has maintained a force of two or three warships in the gulf that are charged with watching over British shipping through the Strait of Hormuz. The ships do not travel to the northern reaches of the waterway.

Mellors told reporters that several considerations contributed to Britain's rejection. British mine sweepers are all assigned to home or Atlantic waters and could not arrive in the gulf "for weeks," giving Iran plenty of lead time to lay more mines, he said, adding that the mine sweepers would also require protection from the "limited resources" of the Royal Navy.

"We never, ever take lightly a request from the United States," he said. "Over events like Libya, we have been prepared to bite some very hard bullets. On this occasion, we have to take a different view of where we think the principal benefit would lie."

He said that "the U.S. government will understand that our commitment is a very real one," including both the existing naval patrol and Britain's agreement last week to allow U.S. Marines and mine-sweeping helicopters to stop at a British naval base on the island of Diego Garcia in the Indian Ocean. The helicopters are to be sent aboard an American C-5 military cargo plane to Diego Garcia for transfer to the helicopter carrier USS Guadalcanal, which will then sail into the Persian Gulf.

In Washington, Pentagon sources said today that the U.S. Navy is preparing to send another team of mine-hunting experts and equipment to the Persian Gulf.

Sources said the contingent, which is based in Charleston, S.C., would work in conjunction with the mine-sweeping and mine-hunting operations of the eight Sea Stallion helicopters being ordered to the Persian Gulf. About 250 pilots, specialists and support workers will accompany that operation.

The Pentagon has also ordered several small coastal mine sweepers to the gulf from their base in Charleston. The MSB-class boats will be delivered aboard a larger ship, which will take about a month to make the trip, military officials said today.

Also in Washington today, the Defense Department issued a casualty list of American servicemen who were aboard the Sea King helicopter that crashed in the Persian Gulf Thursday. It listed the helicopter copilot, Navy Lt. j.g. James F. Lazevnick, 25, of Paulsboro, N.J., as the only serviceman known to have been killed in the crash. The office of Rep. Roy Dyson, (D-Md.), said Lazevnick had recently lived in Waldorf.

The Defense Department listed three servicemen as still missing: the helicopter pilot, Navy Lt. William E. Ramsburg, 31, of Scotland, S.D.; Radioman 2nd Class Albert B. Duparl, of Saint Croix, Virgin Islands; and Air Force Lt. Col. Horace S. Gentle, 44, of Moresville, N.C.

Officials in London are known to have been irritated that news of the Diego Garcia agreement was leaked to the U.S. media. Mellors declined to comment on the agreement, except to say that it provides "an answer to the question of what the hell the Brits are doing," an apparent reference to criticism within the U.S. Congress of allied reluctance to help police the gulf.

One administration official predicted today that the reaction in Congress "will not be a happy one. There is a difference," he said, "between asking the allies for rhetorical support and saying that we're protecting allied interests, and we need a specific piece of equipment."

Asked about possibly adverse Congressional reaction, Mellors said, "We prefer not to be criticized by anybody. But in the end, we just have to try to do a decent and honorable job here."

The chairman of the British Iranian Group in the House of Commons, Eldon Griffiths, said yesterday that sending Royal Navy mine sweepers into the Gulf would "fly in the face" of diplomatic efforts to reach a solution.

Griffiths, a Conservative member of Parliament and strong Thatcher supporter, said "sadly" many Americans would see the refusal as a snub. But he added: "I believe the British government is right in saying that the U.N. resolution should be given more chance to work before we escalate the military presence."

{In Tehran, President Ali Khamenei warned during Friday prayers that "America and the countries which are trying to create a crisis in the gulf will be the first victims of such a crisis," the German news agency Deutsche-Press Agentur reported.

{"After a month of empty words, the whole world understands the comic nature of the threats being made, and America has retreated with an injured pride," he said.} Washington Post staff writers Don Oberdorfer and Molly Moore contributed to this report.