United Nations, N.Y. — The prospect of a United Nations-negotiated settlement between Argentina and Britain has suddenly dimmed.
A mood of pessimism prevails among diplomats here since the return of British envoy Sir Anthony Parsons from London. A last-minute positive development, such as further concessions to Argentina, cannot be ruled out.
But Britain has not answered concessions made by Argentina last week with concessions of its own. In fact, according to sources here, it has toughened its stance and seems to be willing to settle the crisis peacefully only as long as it is basically on its own terms.
''Britain now sees the UN plan as providing an honorable exit for Argentina, but not for itself as well,'' says a well-informed and impartial Latin American diplomat.
The Perez de Cuellar plan was evenhanded and based on a ''no winners, no losers,'' formula. If one side insists on being the clear winner, the plan collapses, according to sources familiar with its basic ingredients.
Britain's insistence on the islands' legislative, executive, and municipal councils playing a major role in the UN's temporary adminstration signals to Argentina British intentions of reestablishing sovereignty over the islands rather than allowing that issue to be settled at the negotiating table.
Speculation among high-level and hard-nosed experts as to what might be prompting Britain to go after a victory, rather than to seek a tie, center on three points:
* Domestic politics and Mrs. Thatcher's preferred Churchillian posture. ''At a time when her public support has been rapidly eroding as a result of her economic policies, by inflicting a lesson on Argentina she may hope to become a British Joan of Arc, a folk hero,'' says a European ambassador.
* The strategic location of the Falklands. As a port of call for ships moving around Cape Horn, sailing from South Africa to South America, and as a springboard for future attempts to explore the mineral wealth of Antarctica, Britain (with possible discreet encouragement from the United States), may be reluctant to give up its control over this geopolitical trump card.
* The dissuasive value of recapturing the Falklands by force. Britain (again possibly with discreet approval of its allies) might want to demonstrate that any of the NATO allies is still in a position to sufficient power 8,000 miles from home to cut a third-world country down to size, even one as technologically and culturally developed as Argentina.
''This would show that the UN Security Council setup (with its five veto powers) still adequately reflects the true world situation in terms of power. Argentina might draw a lesson from this defeat: It could decide to go nuclear for political reasons, in order to assert by might if not by right, its place in the sun,'' says a senior Western European diplomat.
Legally, Britain's position is still strong inasmuch as it can claim to simply seek to regain stolen property. But politically, its position at the Security Council, after an attempted invasion of the islands, would be much weaker than it was on April 2, according to informed sources. The Soviet Union and the nonaligned countries would not condone Britain's ''attempt to take the law into its own hands.''