British sources said today that any peace agreement drawn up in the United Nations to resolve the Falklands crisis must contain a clause "which clearly precludes prejudging the outcome" of negotiations on the future of the islands.

The British said the clause is crucial to prevent "a thinly disguised, delayed handover of the islands to Argentine sovereignty and possession." Britain wants "a genuine interim arrangement and negotiations where the eventual outcome could be anything," the sources said.

The five-day old U.N. effort to arrange peace negotiations still seemed to be hung up over the conflicting Argentine and British demands on the groundrules for handling the sovereignty issue.

Argentine officials have said publicly that the talks under a U.N. representative must contain "precautions" that "inexorably lead to the recognition of our sovereignty." U.N. officials said Argentina had made the same points privately to Secretary General Javier Perez de Cuellar.

Perez de Cuellar told reporters this morning he had sought to bypass the sovereignty problem by "trying to work on the basis of a procedural mechanism."

The mechanism includes a cease-fire, mutual withdrawal of forces, an interim U.N. administration over the Falklands and negotiations on the sovereignty issue under a U.N. representative, with a six-month target date to reach agreement.

The British sources said today that the details of the mechanism are either agreed upon by both sides or easily soluble.

U.N. officials said the question of sovereignty arose indirectly, however, in discussing the structure of the upcoming negotiations. It became clear that on that issue, the two sides would show little of the flexibility they had exhibited until now on the other issues.

In London, British Prime Minister Margaret Thatcher told Parliament that Argentina cannot be guaranteed that it will be ceded sovereignty of the islands at the end of negotiations, Washington Post correspondent Jay Ross reported.

Thatcher said the requirement was one of two "fundamental principles which we cannot fudge in any way." In addition, she said, a cease-fire "must be accompanied by a withdrawal to a specific timetable--and in a comparatively short time."

She did not refer, as she has frequently in the past, to the "paramountcy" of the wishes of the islanders, who have repeatedly opted for British control, or to the form of an interim administration after withdrawal of Argentine troops.

The British sources declined to specify the language they sought. "It has to be something that can be clearly understood by the British government, Parliament and the man on the omnibus to Clapham a lower middle-class section of London ," the sources said.

The British source seemed to rule out any formula that would satisfy British needs and still let Argentina claim that the agreement will end with its control over the islands. "This is one fundamental point we cannot finesse. We cannot have constructive ambiguity because we have to make sure the talks don't break down in the first week over it," the source said.

Argentine negotiator Enrique Ros arrived for the first of two appointments with Perez de Cuellar today saying he had brought "more ideas."

British Ambassador Anthony Parsons also met twice with Perez de Cuellar and said afterward, "I got some interesting proposals from the secretary general this evening. I think we are making progress again."

Perez de Cuellar said as he left the U.N. for the night, "I think today we made some progress. I am rather encouraged . . . of course, we are still far from a solution. I think some more days are needed . . . ."