The British government is firmly opposed to negotiations on the future of the Falkland Islands, whatever the makeup of the new Argentine junta and regardless of mounting pressure from its American and European allies to begin talks, British officials said today.

Prime Minister Margaret Thatcher recognizes that Britain's recalcitrance will probably strain its relations with the United States and the European Community for a time, the sources said, but she nevertheless is adamant in her refusal to bargain for the foreseeable future.

"We did not win this war just to hand over the Falklands a few weeks later," one official said.

Both the Reagan administration and the European Community, which supported Britain in its conflict with Argentina over sovereignty in the Falklands, are eager to repair ties with Latin America, including Argentina, as quickly as possible. British officials expect their allies to lift most of their economic sanctions against Argentina shortly, perhaps as early as a community meeting this weekend.

The Thatcher government is expected to ask at that session that the Europeans at least continue their embargo on arms shipments to Argentina and make the lifting of economic sanctions contingent on an end to fighting in the area.

Since the fall of Port Stanley in the Falklands on Monday, the British have been seeking explicit assurance from Argentina that it will cease all military activities related to the islands. Officials here expressed hope that yesterday's ouster of Argentine president Leopoldo Galtieri may improve the chance that such a peace pledge will be forthcoming.

Sources said that Britain would now be satisfied with a tacit Argentine promise passed through a third party, possibly Switzerland or Brazil, which have been representing the diplomatic interests of the two countries, instead of a formal declaration that hostilities are ended.

Britain would not consider this a first step toward resumption of the bargaining over the future of the Falklands. These talks ended when Argentina seized the islands April 2, touching off its undeclared war with Britain.

"There is no connection," one senior official said.

Argentina would gain two things in return for its assurance of non-belligerence, sources said: return of several thousand prisoners Britain now intends to hold indefinitely and the lifting of various naval embargoes on the region that severely limit Argentine shipping. But that is all Britain is prepared to give up at this time, the sources said.

About half of the estimated 11,000 Argentine prisoners now on the islands have been put aboard British ships to return to the mainland. Argentina, which earlier had refused to accept the returning POWs, sent word after Galtieri's fall that it would provide safe passage to the coast for the British vessels.

The requisitioned cruise liner Canberra and ferry Norland set sail from the Falklands carrying about 5,500 Argentine prisoners, news agencies reported. The ships were bound for Puerto Madryn in southern Argentina.

The turnover of prisoners is regarded here as a humanitarian gesture--as well as a practical one, since Britain has no means of caring for so many captives on the Falklands. Sources said the prisoner issue is not the equivalent of a military disengagement that would precede political bargaining.

While Thatcher refuses even to discuss a timetable for sovereignty talks, diplomatic sources said that she might be more flexible in a few months if she is not too heavily pressured by her allies now.

"It would make sense to let matters cool over the summer and then see how she feels," one knowledgeable official said.