Britain's support of the U.S. air attack against Libya, and its agreement to let American bombers take off from here, was a one-time policy, Prime Minister Margaret Thatcher said today. The United States would have to ask again if it contemplated similar action in the future.

"It is clearly understood between President Reagan and myself," Thatcher told Parliament, "that if there were any question of using U.S. aircraft based in this country in a further action, that would be the subject of a new approach" to her government.

In a formal parliamentary debate in which speaker after speaker criticized her decision, Thatcher acknowledged the risks of possible terrorist retaliation in Britain and strains on the country's relations with most of its allies who disagree with her.

The reasons for her decision, she said, included Libya's proven complicity in terrorism -- not only in recent bombing incidents but also in past material support to Britain's enemy, the Irish Republican Army in Northern Ireland.

Peaceful attempts to persuade Libyan leader Muammar Qaddafi to desist had failed, she said. When the United States -- which Thatcher described as Britain's "greatest ally," -- proposed military action, she said she approved the plan both on its merits and out of solidarity.

There was general agreement here that it will take some time to determine whether Thatcher has suffered any lasting domestic damage because of her decision. So far, she has given little indication that she fears a political backlash.

"I think she's used to it," a senior Thatcher aide said of the prime minister's critics, and her customarily unflappable way of dealing with them. "She doesn't give a damn. What she's concerned about is whether [the decision] was correct."

Majority approval of her action -- and a reflection of her Conservative Party's 140-strong majority in the House of Commons -- was given in a 325 to 206 vote following the session. Although a handful abstained -- including former Conservative prime minister Edward Heath -- and one party backbencher voted against her, senior government officials and the party rank and file were generally supportive during the debate.

But numerous speakers during the seven-hour session echoed the editorial opinions printed in the majority of the country's majors newspapers that Thatcher had been wrong.

In the first face-to-face opinion poll on the issue, taken by Independent Television News and the Harris Co., 59 percent disapproved of the U.S. attack, while 32 percent approved. Asked their opinion of Thatcher's cooperation, 68 percent were against it, while 28 percent approved.

Criticism of the attack, and Britain's role in it, fell into several general categories.

It was likely to increase rather than decrease Libyan terrorism: "The use of such force does not punish" or prevent terrorism, said opposition Labor Party leader Neil Kinnock. "Indeed, the use of such force is much more likely to provoke and expand terrorism."

It reinforced the impression that Thatcher is incapable of separating British from American interests: "The government made a severe error of judgment," said Liberal Party leader David Steel. The British people want a leader "who will think it is conceivable occasionally to say 'no' to the occupant of the White House."

The decision was inconsistent with prior British, and even Thatcher, policy on terrorism: Calling the attack "a futile, deplorable" act, the respected Financial Times said the British government "has compromised its position in ways which are logically and politically untenable." Britain, it said, "has never believed terrorism can be defeated by military attacks and it does not believe it now.

It made Britain and Britons abroad -- particularly the 5,000 living in Libya -- a front-line target for terrorism: Two British warships were reported heading east in the Mediterranean today from Gibraltar in case they are needed to evacuate British citizens in Libya.

In response to this last charge, Thatcher argued that Britain already is in the front lines. It is subject not only to IRA attacks, but also was a victim of Libyan terrorism, when a London policewoman was killed outside the Libyan Embassy in 1984.