President Reagan's unexpected decision last week to end his policy of observing SALT agreements on offensive arms was influenced by last-minute appeals from administration conservatives who feared they had lost the battle on this contentious issue, according to U.S. officials.

Those appeals reinforced other factors that led to Reagan's decision, including doubts about whether there will be a superpower summit this year, displeasure among congressional conservatives about Reagan's strategic arms policies and a White House conviction that U.S. allies were not particularly alarmed at the prospect of a shift in Washington's policy of compliance, informed officials said.

Only six weeks ago, in mid-April, Reagan had sent two senior emissaries to U.S. allies with a tentative decision leaving open the issue of U.S. observance of the arms accord limits at least until late this year. As recently as mid-May after the Tokyo summit, a proposed draft of a presidential statement circulated by the White House did not firmly declare the end of U.S. compliance with the 1972 and 1979 SALT agreements, which were the heart of U.S.-Soviet relations under Presidents Nixon, Ford and Carter.

The Reagan decision announced Tuesday, which prompted protests by western allies to Secretary of State George P. Shultz at a North Atlantic Treaty Organization meeting and a Soviet government warning, has been described by proponents and opponents as among the most important of his administration. According to several senior officials, however, it was taken by Reagan without any meeting on the subject with his key advisers in the crucial final weeks when it was shaped.

The final arguments that seem to have tipped the scales against SALT were contained in written memos to Reagan following circulation within the government two weeks ago of the proposed presidential statement. One official described that draft as "pretty mushy stuff. . . a bowl of porridge" without a clear U.S. position.

The policy announced last week ended a pledge Reagan made early in his first term not to "undercut" the SALT II limits on offensive strategic weapons. Nevertheless, the United States remains in compliance with the unratified SALT II pact at least until the fall because Reagan has decided to take two older Poseidon submarines out of service. This is now described as a pragmatic decision not connected to SALT II limits.

The White House also announced that the United States will not be bound by 1972 SALT I treaty restrictions, some of which still apply to the superpower arsenals. A third major accord between Washington and Moscow, the Anti-Ballistic Missile Treaty of 1972, was not directly addressed in last week's announcement.

Secretary of Defense Caspar W. Weinberger, CIA Director William J. Casey, Arms Control and Disarmament Agency Director Kenneth L. Adelman, Arms Control Adviser Edward L. Rowny and other conservatives were on the winning side of the final battle, according to officials. On the losing side, the sources said, were Shultz and Arms Control Adviser Paul H. Nitze, who nevertheless have publicly supported Reagan's decision in recent days.

Key personnel shifts may also have played an important role. Changes in the Joint Chiefs of Staff converted its previous support for SALT II as advantageous to U.S. national security to the neutral view that adherence or non-adherence with the U.S.-Soviet agreements is militarily "a wash."

And the replacement of Robert C. McFarlane as White House national security affairs adviser by John M. Poindexter, at the minimum, removed a veteran backer of arms control from that sensitive post. Poindexter's views on this subject are unknown even to some who have participated in lengthy discussions over which he presided.

Reagan, who opposed the 1979 SALT II treaty as "fatally flawed" while a presidential candidate, has been living uncomfortably with it ever since.

Civilian leaders in the Pentagon and other administration conservatives were unhappy with Reagan's observance of the accord but were initially unable to persuade the president to risk the international and domestic uproar expected to follow the junking of the SALT agreements.

What eventually turned the tables was the issue of Soviet non-compliance. The administration, under prodding from Congress, charged that the Soviets were not upholding their part of the mutual "no-undercut" policy, especially by deploying two "new" missiles instead of one as permitted by SALT II, and by encoding electronic missile data -- known as telemetry -- beyond what was allowed. The Soviets denied the charges in both cases and many experts have said the charges are debatable.

Last June 10, following a contentious round of internal policy-making, Reagan surprised many people by announcing that despite his charges of Soviet non-compliance, he would "go the extra mile" for arms control by dismantling a Poseidon nuclear submarine. This was necessary to keep U.S. forces within the numerical limits of SALT II as a more powerful new Trident nuclear submarine went to sea.

A key factor in that decision, according to a policy-maker who was involved, was the prospect of a summit meeting later in the year with the newly installed Soviet leader, Mikhail Gorbachev. "All the arguments against SALT sound good, Mr. President, but do you really want your allies dumping on you when you go into the room with Gorbachev?" a senior official reportedly asked during the 1985 decision-making.

The impending deployment this spring of another new Trident required another decision about whether to compensate for this by dismantling additional older Poseidon subs. As in the previous case, the Navy preferred to dismantle two 20-year-old Poseidons, whose nuclear reactors had come to the end of their useful life anyway, rather than spend several hundred million dollars to keep them afloat.

Following a National Security Council meeting on the issue April 16, Reagan decided to order the two older submarines cut up as the Navy recommended. But he also expressed renewed concern about continued U.S. compliance with SALT while Soviet violations persist. The president reportedly accepted recommendations that the United States respond to the violations by again asking Congress for 50 more MX missiles and by accelerating work on an advanced type of nuclear-armed cruise missile and on a second "new type" of U.S. land-based missile, possibly the single-warhead Midgetman.

Reagan was aware that around November of this year air-launched cruise missiles on additional B52 bombers would once again take U.S. forces beyond the SALT II numerical limits in the absence of further compensating measures, such as retirement of more old submarines or missiles.

In instructions to Nitze and Rowny, who were sent to consult U.S. allies following the April 16 meeting, Reagan indicated doubts about continuing SALT adherence in November but, according to several officials, made no commitment other than to say he would consider all relevant factors at the time, including U.S. military requirements and the Soviet compliance record.

These instructions, which were reflected in the mid-May "draft presidential statement," were seen by U.S. allies and other SALT II backers as a reprieve for the treaty, at least until November. Officials speculate that this may be among the reasons top allied leaders reportedly did not press Reagan hard on the issue during the Tokyo summit last month.

Concern about the Soviet reaction was diminished by the dimming prospects for a summit this year, especially when Moscow canceled a scheduled mid-May summit planning session in reaction to the U.S. bombing raid on Libya.

A CIA study last June reportedly estimated that the Soviets could rapidly expand their strategic force to 19,000 nuclear warheads from around 10,000 if released from the SALT II restrictions. But treaty opponents argue that such a move would be unlikely because it would be expensive and simply add to Moscow's overkill capability against U.S. targets.

Also, a decision to jettison U.S. compliance with SALT was seen as certain to have domestic repercussions. But sticking with SALT was irksome to Reagan's conservative allies and pleasing to his liberal foes, officials said.

"It was obvious where Reagan was coming from all along," said an official in favor of junking the SALT restrictions, "but he doesn't like to do instinctive things that have high costs to him." This time around, the official added, the immediate cost of abandoning the treaty seemed less than before.