British Prime Minister Margaret Thatcher said yesterday that Britain will fully support any attempt by the United States to obtain a United Nations resolution calling for economic sanctions against Iran.

Emerging from a more than two-hour meting with the president at the White House, Thatcher said she had "indicated very clearly to the president that when the United States wishes to go to the Security Council for further powers . . . Great Britain will be the first to support him in his endeavor."

"You would expect nothing less and you will receive nothing less but our full support," she added.

The explicit pledge of British backing was welcome news to administration officials who have been attempting to line up the support of U.S. allies for additional measures to pressure Iranian authorities to free the 50 American hostages in Tehran.

A U.N. Security Council resolution calling for economic sanctions against Iran is one of the measures discussed with European leaders last week by Secretary of State Cyrus R. Vance. But White House press secretary Jody Powell stressed yesterday that other avenues are open to the United States and may be pursued before the administration takes its case to the Security Council again.

Powell also warned that Iran will be forced to pay "an increasingly higher price" if it persists in holding the hostages, suggesting that the severity of U.S. countermeasures will grow as the crisis continues.

While a U.N. resolution continued as one likely strategy, informed sources said the administration also is discussing economic sanctions that can be taken individually by trading partners of Iran. The actions probably will not be announced, but simply put into effect, the sources said.

At a news conference after her meeting with the president, Thatcher made clear her preference for collective action under a mandate from the United Nations rather than individual measures by the United States and its allies.

She noted that, absent a U.N. resolution, the laws of Britain and other countries limit the economic measures that could be imposed on Iran, making U.N. approval "very important."

Thatcher also stressed the importance of the United States first negotiating with other Security Council members to assure passage of a sanctions resolution and to settle beforehand what kind of sanctions would be imposed.

Key to such negotiations is the attitude of the Soviet Union, which could veto a Security Council resolution. The United States is sounding out the Soviets on the matter of sanctions.

The crisis in Iran dominated Thatcher's first state visit to the United States, although other subjects, including the British-led Zimbabwe-Rhodesia settlement and SALT II, were discussed.

Thatcher said the talks went "extremely well," and Carter said "there are no differences between us that cause any concern among Americans or people who live in Great Britain."

The president, greeting Thatcher at the White House yesterday morning, also stressed the support the United States has received from Britain during the Iranian crisis.

"From the very first moment when American hostages were captured and held illegally in Iran, your government has been in the forefront of those who have helped us in every way," he said. "Both privately and through diplomatic means, and through public means, you have been staunch allies, staunch friends, staunch supporters of ours."

Administration sources continued to say that the United States is likely to call soon for concerted international economic action against Iran, possibly in the form of U.N. sanctions against most forms of trade until the hostages are released.

State Department spokesman Hodding Carter said the United States is evaluating its next steps daily, suggesting that a decision is not far off.

The State Department continued to take an unyielding stance against an international tribunal to investigate the shah's regime, even if the hostages were "witnesses" rather than defendants, as suggested by Foreign Minister Sadegh Ghotbzadeh. One theory, not now accepted by U.S. officials, is that Iran intends such a tribunal to be a device to justify releasing the hostages without losing face.

Spokesman Carter said use of the hostages in any tribunal "would be repugnant . . . a cruel invasion of their privacy and their rights." The United States has asked Americans not to become involved in a tribunal or formal study of the shah's regime until the hostages are freed, taking the position that their release is a necessary first step.

Among the factors being closely scrutinized by administration officials is the reaction of various Iranian power centers to the departure of the deposed shah, Mohammed Reza Pahlavi, from the United States.

The United States had been informed by diplomats in touch with Iran that the shah's departure appeared to be a prerequisite for movement toward the hostages' release even though quite different signals were coming from the student militants holding the Americans in the embassy compound in Tehran.

It is still too early to tell what the full effect will be of Pahlavi's move to Panama, according to U.S. sources. But they found it interesting -- and not necessarily discouraging -- that the Iranian Revolutionary Council, student militants and government radio all portrayed Pahlavi's departure as an American defeat instead of describing it as a U.S. subterfuge or provocation. Until recently, Iran's central demand has been that the United States extradite the former shah.