Prime Minister Margaret Thatcher said today that Britain would do "everything possible" to help the United States secure the release of American hostages held in Iran, but added that British moves would be made in consultation with other European nations.

The most vocal support so far for U.S. efforts to resolve the Iran crisis have come from Britain and West Germany. France has shown the most reluctance to join in tough economic sanction's against Iran and other members of the European Common Market have kept a relatively low profile.

The EEC states and Japan decided to recall their ambassadors in Iran after the envoys met on Saturday with Iranian President Abol Hassan Bani-Sadr.

Thatcher said in Parliament that she expected the "necessary decisions" would be taken at a meeting of the foreign ministers of the nine Common Market nations next Monday and Tuesday. She did not specify what measures were under consideration, however.

The Carter administration has called on U.S. allies to join the United States in imposing tough economic sanctions on Iran. In addition, the president told European television correspondents he has asked the allies to take "strong diplomatic moves" by a "specific date" next month if the sanctions have not won the hostages' release by then.

Deputy Secretary of State Warren Christopher said Sunday that the administration hoped for allied moves on sanctions as early as April 21. Carter's deadline for the allies to take diplomatic action if the sanctions failed was not known, but unconfirmed reports in Washington put it in the first part of May.

Thatcher said Britain "will do everything possible to help the United States." She noted that "diplomatic action has not so far succeeded" and said the next steps to be taken by Britain and other Common Market nations "must be either political or economic action or both."

The prime minister indicated that Common Market rules may authorize group action to restrict trade with Iran -- legal authority that officials here believe Britain lacks without a United Nations order imposing economic sanctions. EEC officials studying the Common Market's rules in Brussels believe they have found such authorization in the market's common trade policies.

But the Common Market foreign ministers must ratify this interpretation next week and then decide how far to go in further reducing the EEC's already dwindling trade with Iran in support of the U.S. economic sanctions imposed by Carter last week.

European leaders reportedly feel next week's meeting must produce some demonstration of solidarity with the United States. Thatcher said today that Carter had not set a specific deadline for allied action. That statement appeared to contradict the text of Carter's interview with the European television correspondents, which was made public Sunday.

Among measures reported to be still under discussion are Common Market embargoes on specific kinds of trade with Iran, including badly needed spare parts for its oil industry and transportation system. As Thatcher pointed out today, Britain and most other West European nations already have cut off sales of military equipment and spare parts to Iran.

West Germany still appears ready to go furthest in imposing economic sanctions on Iran. West German government spokesman Klaus Boelling said in Bonn today that "We will do everything in our power to ensure a common EEC position."

Britain and Italy appear ready to join in an embargo of certain kinds of trade with Iran, but they have been reluctant to join in sweeping trade and banking sanctions. Officials here are concerned about damaging confidence in Britain's international banking business, while officials in Rome worry about the 1,600 Italians working on more than $3 billion worth of construction projects in Iran.

Both Thatcher and the opposition Labor Party Leader, James Callaghan, expressed concern in Parliament today with Carter might take military action against Iran. "Any contemplation of that might be an extremely serious step indeed," Thatcher said.