Belgium, opening a crack in NATO's support for swift installation of medium-range nuclear missiles in Europe, has put President Reagan on notice that it will not begin deploying missiles in March as planned and could delay a decision on deployment until next year.

Reagan met with Belgian Prime Minister Wilfried Martens at the White House Monday but failed to persuade him to adhere to a March deployment schedule. Instead, Martens' insistence that he needs more time to decide appeared to undercut public assertions by U.S. officials that they remain "optimistic" about Belgium's acceptance of the missiles.

U.S. officials yesterday tried to put the best face on the situation by contending that there had never been a firm decision to begin Belgian deployment in March. They noted that Martens publicly had told Reagan that Belgium remains committed to NATO's decision to deploy the missiles by the end of 1987 unless the Soviets agree to reduce their arsenal of SS20 medium-range missiles.

In private, the U.S. officials conceded that, while they understand Martens' need to buy time, it appears to show weakness within the North Atlantic Treaty Organization at a crucial time in East-West relations. The U.S. sought fast action by Belgium to show that America's NATO partners are united on the so-called Euromissile question, thereby strengthening the U.S. hand as it prepares for new arms control negotiations with the Soviets.

At their Geneva meeting last week, Secretary of State George P. Shultz and Soviet Foreign Minister Andrei A. Gromyko agreed that medium-range missiles in Europe would be one of the three types of weapons covered by the talks. That gave heightened significance to Reagan's discussions with Martens because they marked the first post-Geneva test of NATO's resolve to press ahead with deployment in Western Europe of 572 U.S.-made cruise and Pershing II missiles.

However, the best Reagan could get was Martens' promise that his government will decide its next move by the end of March. Belgian officials said later that even if Martens does set a deployment timetable then, it will require time-consuming scrutiny and debate in the Belgian parliament. The officials added there is a strong possibility that Martens will decide to postpone a decision until after the Belgian national elections scheduled for December.

"It is our belief that the government of Belgium will live up to its NATO commitment," White House spokesman Larry Speakes said in an effort to tamp down speculation that Martens, facing strong domestic opposition from antinuclear forces, is pulling Belgium back from participation in the Euromissile program.

The pessimism was not dispelled by Martens' ambiguous responses during an interview yesterday with CBS News. He said his government is continuing consultations with its NATO allies and promised to make a decision "before the end of March" on "a tight timetable for the starting point of the deployment," but he would not specify when the timetable might go into effect.

"This decision could involve deployment, and it could take place in March," Speakes said, noting that site preparations have gone ahead for the first 16 of the 48 cruise missiles scheduled for emplacement in Belgium.

However, Belgian Foreign Minister Leo Tindemans, in a meeting here with U.S. reporters, tacitly acknowledged that deployment in March is highly unlikely. Tindemans said that Martens' coalition government of four center-right parties is wrestling with a choice between two courses, both of which would mean delays in implementation.

One option outlined by Tindemans would see the Belgian government set a deployment timetable and then seek a vote of confidence in parliament. He noted that since the coalition has only a six-seat parliamentary majority, that course could cause the fall of the government and precipitate early elections.

The other course, Tindemans said, would be to postpone a deployment decision until after the December elections, thereby making Belgium's willingness to accept the missiles the central campaign issue.

Three NATO countries -- Britain, West Germany and Italy -- already have started positioning their share of the missiles. But since NATO's 1979 decision to accept the missiles, fierce antinuclear sentiment has stalled progress on getting them into Belgium and the Netherlands.

Despite denials yesterday by Martens and U.S. officials that March had been agreed upon as the deployment date, Belgian Defense Minister Freddy Vreven told parliament Nov. 6 that the first missiles would arrive there in March. He added that a formal government approval would be required, but Belgian officials said then they did not expect a change of position.

However, Martens subsequently came under pressure from elements of his Flemish Christian Democratic Party, who argued that Belgium first should see what progress resulted from new U.S.-Soviet talks. That forced the prime minister to declare on Nov. 30 that he would put deployment on hold until he had conferred with Reagan and other NATO leaders and would make a decision during the first three months of this year.

Special correspondent Steven J. Dryden reported from Brussels:

Political supporters of Prime Minister Martens today praised the results of his meeting with President Reagan, indicating they believed that Belgium would now delay its deployment of cruise nuclear missiles.

Martens' party has urged him to postpone installation, scheduled by NATO to begin in March.

Christian Democratic leaders reportedly believe that deployment in March could cause the defeat of the government in the forthcoming December elections, although publicly they have said the postponement is necessary to encourage the new U.S.-Soviet arms talks.

Frank Swaelen, president of the Flemish Christian Democrats, praised Martens' position: "This is undoubtedly a very positive outcome. It recognizes the need to give the U.S.-Soviet Geneva talks a chance."

Guy Verhofstad, leader of the Flemish Liberal Party, another member of the Martens coalition government, said of the Belgian position: "This is not a unilateral rejection of deployment. Belgium remains a faithful alliance member."

But the major political opposition to the government, the Flemish Socialist Party, denounced what it termed a "so-called compromise" between Martens and Reagan.

"It is only by not installing the missiles in our country that we can make our contribution to halting the arms race," said Karel Van Miert, the party leader.

One of the main Belgian antimissile groups, VAKA, said the government had made a "blatant maneuver to overcome internal problems and survive the next elections." The government, VAKA added, is only debating when to deploy, "when the fact is that the majority of Belgians do not want deployment, now or in 1987."

Martens undoubtedly will be under pressure from political supporters to put off the beginning of deployment until 1986, after the elections, western diplomats said.

His supporters fear that because of the unpopularity of the government's austerity program, and other political problems, it would be best at election time to have the volatile missile issue out of the way.