Secretary General Javier Perez de Cuellar informed the U.N. Security Council tonight that his final effort to end the Falklands crisis had collapsed.

He conceded publicly what had been clear hours before--that Britain and Argentina are not yet prepared to bridge the differences between them. U.N. aides said Perez de Cuellar had made the decision after final conversations with both the British and Argentine negotiators.

The Peruvian secretary general expressed his commitment "to the search for a lasting resolution of this problem." But he said he would not seek to initiate a second mediation effort unless he were asked to do so by the Security Council.

The council members are expected to consult Friday on what they can now do. The Soviet Union, Panama and other backers of Argentina had been urging a public debate on the question, and had been restrained only by the secretary general's ongoing mediation effort.

In his letter to council president Ling Qing of China, Perez de Cuellar said substantial progress had been made but "the necessary accommodations which were still needed have not been forthcoming. In these circumstances the efforts in which I have been engaged do not offer the present prospect of bringing about an end to the crisis nor of preventing the intensification of the conflict."

These differences involve specifically the distances and timing of mutual military pullbacks from the islands, the role of the islanders under a proposed interim U.N. administration, what would happen to the U.N. presence if negotiations on the sovereignty issue have not been completed by a target date of the end of 1982, and the status of the outlying South Georgia and South Sandwich Islands.

Perez de Cuellar said these differences are not irreconcilable, suggesting what most diplomats here argue--that the basic failure lay not in the details of disagreement, but in the absence of a will to agree.

The secretary general was careful not to blame either side for the collapse of his three-week effort. The Argentine response to his final proposals, put forth last night, arrived late tonight.

Earlier, the British ambassador, Sir Anthony Parsons, delivered his government's ambivalent reply to Perez de Cuellar, which was in essence that Britain welcomes the secretary general's continuing efforts but will await Argentine concessions.

Each nation published detailed defenses of the positions it had taken during the three-week-long U.N. negotiating process, seeking to avoid blame for the diplomatic stalemate.

The peace effort by Perez de Cuellar that collapsed today began on April 30--four weeks after the Argentine invasion and on the day that Secretary of State Alexander M. Haig announced abandonment of his own mediation shuttle.

The secretary general met the British and Argentine foreign ministers to offer his framework for a settlement--a cease-fire, mutual withdrawals, and temporary U.N. administration of the islands while talks under a U.N. representative resolve the sovereignty issue.

The British reacted coolly until, on May 7, Argentina closed down the last alternative channel, involving the United States and Peru, and the U.N. talks began in earnest.

Perez de Cuellar met daily--sometinmes twice a day--with Parsons and then with Argentina's Deputy Foreign Minister Enrique Ros, asking each side to suggest ways of overcoming differences, and sometimes offering his own ideas.

That first weekend, both sides accepted the link between a cease-fire and the staged withdrawal of forces. But then came the first stalemate, over Argentine insistence that the "context" of the agreement must make clear that Argentina will "inexorably" attain sovereignty over the islands.

The British rejected this, insisting that the outcome of the final talks must not be "prejudged." But British sources told reporters they believed all other issues could be resolved easily.

Argentina agreed on May 11 that sovereignty was its "objective," and not a precondition, which appeared to defuse this issue. But on May 13, after members of British Prime Minister Margaret Thatcher's party raised fears of a "diplomatic sellout," the British position toughened on at least two key issues--withdrawal arrangements and the islanders' role during the interim administration.

The British sources had initially said that the timing and distance of the pullbacks was "not a major issue." But the latest British proposal, submitted Monday, specified that the battle fleet could remain within 150 miles of the islands, and that both sides should withdraw within two weeks.

Argentina, in its response delivered to the British on Wednesday, sought a 30-day limit and the return of the British ships to their home ports.

The British sources had initially said that during the U.N. administration there would have to be "some means to ascertain the wishes" of the islanders, but details of the U.N. regime were "not a major problem."

After domestic pressures mounted on this issue, Britain insisted on having the U.N. administrator act "in consultation with" the executive and legislative councils that form the island's government under British rule--with the addtiion of an Argentine resident to each council.

Argentina opposed a role for the councils on the ground that they would weight the sovereignty talks in Britain's favor. The Argentines also called for free access to the islands for Argentine nationals during the interim agreement--a new demand that could change the demography of the Falklands, as the British noted.

Two other major issues also remained unresolved. Britain insisted from the start that if there were no agreement by the target date--the end of this year--the talks and the U.N. administration should continue, perhaps through monthly renewals. In its latest proposal, Britain called for an open-ended U.N. presence, without the need for periodic renewal, until a definitive agreement on sovereignty is reached.

Argentina sought a specific termination date for the talks, with no explicit provision for renewing the U.N. administration. It called for bringing the sovereignty issue to the General Assembly--where the majority favors their view--if the talks stall.

Another significant difference emerged only late in the talks. British sources said they had worked under the assumption that the agreement included only the Falklands and not South Georgia and the South Sandwich Islands, which would remain under British rule.

In the end, Britain submitted a side letter to the U.N. excluding these distant Falkland dependencies, and Argentina insisted that they had always been and must still be an integral part of the Falklands, thus requiring a British pullout.

The British and Argentine final position papers, unlike earlier communications, were transmitted in full to the other side by the secretary general. Previously, he had given verbal summaries of the positions, interspersed with his own questions and ideas.

All three participants recognized when the two papers were compared that positions had hardened, with broad gaps remaining. Yesterday, after placing phone calls to Thatcher and President Leopoldo Galtieri, Perez de Cuellar for the first time offered his own compromise suggestions on how to bridge the gaps.

The content remains unknown, but now that Britain has authorized a military escalation in the war zone, the real question is whether the secretary general waited too long.

His aides say he could not have acted until the final papers were in, and seen to be short of agreement. To move earlier, they say, would have preempted the Argentine government. Once the replies were in, Perez de Cuellar moved expeditiously, they insist.