The superpowers' agreement to remove nuclear missiles from Europe has heightened apprehensions in NATO that budgetary pressures in most member nations will preclude much-needed improvements in the alliance's conventional forces, according to U.S. and allied officials.

For many European leaders, the signing of the treaty in Washington next week will only emphasize the large disparities in the conventional balance between the Warsaw Pact and NATO.

Despite renewed calls for urgent improvements in their conventional combat forces, top leaders of the North Atlantic Treaty Organization concede they are facing dwindling budgets that will leave them with incremental improvements at best. NATO officials and military analysts say there are unlikely to be any major improvements in hardware and force structure for at least a decade.

"It's a very pensive mood," said one high-ranking American NATO official. "There's a psychological fear of the unknown."

European leaders argue, however, that it is the fear of the known that concerns them most, the overwhelming disparities in the combat hardware numbers between the Warsaw Pact and NATO: the 2-to-1 advantage in tanks and helicopters; the 3-to-1 difference in artillery and guns; the 6-to-1 imbalance in surface-to-air missiles; the 5-to-1 advantage in fighter/interceptor aircraft.

NATO forces traditionally have countered the hardware disadvantage with better training, leadership and morale among its troops, superior weapon technology and reliance on nuclear weapons considered more accurate than the Soviet counterparts. In addition, much of the Soviet weaponry is aging and would be less survivable in combat conditions, while many troops, tanks and other combat vehicles would be committed to East European nations and therefore unavailable for the front lines.

New studies in both the United States and Europe have argued that because of these intangible considerations, NATO and Warsaw Pact nations are on a much more equal military footing than the statistics suggest.

Some European leaders believe, however, the hardware advantage may tip the imbalance even further in favor of the Soviets with the elimination of medium- and shorter-range nuclear missiles, the intermediate nuclear forces known as INF.

"The conventional disparity is even worse -- they are able to invade," said Capt. Peter Monte, a West German NATO spokesman echoing the concern of many officials in his government. "That fact is existing even more in light of the INF agreement. . . . To keep up a credible deterence with the remaining forces, they have to be modernized."

Other European leaders, as well as U.S. Defense Secretary Frank C. Carlucci, argue that the conventional military imbalance will be no greater after the INF treaty, but that greater attention is certain to focus on existing disparities. The United States also argues that removal of INF missiles does not deprive NATO of the ability to respond to a Warsaw Pact invasion with nuclear weapons, both tactical weapons in Europe and strategic weapons in the United States and on submarines.

While the wide imbalance in numbers of tanks, aircraft and other hardware is relatively well documented, it is difficult to pinpoint the disparities in troops because virtually every government and agency that provides information counts a different way.

The International Institute for Strategic Studies, whose surveys of relative military strength are considered authoritative by western governments, notes that NATO has 2.3 million troops compared with the Warsaw Pact's 2.2 million stationed from the Atlantic to the Urals. The institute also reports that in wartime NATO could mobilize 149 divisions of active duty and reserve troops compared with the Warsaw Pact's 201 divisions.

NATO has not released new force figures for the past two years because it cannot agree on how to count the troops.

Many NATO officials say they hope to use the atmosphere created by the signing of the INF accord to reduce the bickering among allied nations and speed their efforts to reach an agreement on a proposal for a conventional arms reduction forum with the Soviets. But even if NATO can agree on a proposal, European leaders as well as President Reagan, say they believe a treaty with Moscow is years away.

Much more immediate, say officials on both sides of the Atlantic Ocean, is the need to modernize and improve conventional forces, a process long mired in multiple military bureaucracies slow to initiate change, the budgets of 16 nations with different priorities and the protectionist politics of nations jealously guarding the turf of their defense contractors.

That is where officials say the INF treaty could have the most significant impact.

NATO military officials for years have been slogging through plans to improve coordination of allied armies and air forces, each with their own models of tanks and artillery and aircraft and frequently unable to communicate with each other or coordinate battle plans.

War games have revealed that up to half of NATO aircraft could be shot down by NATO troops unable to identify them. Radio communication on the ground is frequently incompatible. With different nations manufacturing different guns, ammunition cannot be traded among troops, a possible catastrophe in wartime.

Even well-intentioned plans can be foiled by inattention to small details. After the United States persuaded several NATO air forces to use the F4 fighter, they discovered the planes could not be refueled because the fuel nozzles at each European airfield were different.

West German Gen. Wolfgang Altenburg, chairman of NATO's Military Committee, told a meeting of NATO defense ministers in Brussels this week that the allies' highest priorities now should include improving compatible weapons systems, battlefield communications and identification systems to distinguish between "friends and foes" in the air.

Other NATO military specialists say elimination of the shorter- and medium-range nuclear forces will require the allies to pour more money into improving electronic equipment to protect aircraft during wartime penetration across Warsaw Pact borders by fooling enemy radar.

But the jolting reality of declining national budgets frustrates NATO's efforts to improve conventional forces. According to Lord Peter Carrington, secretary general of NATO's military committee, only four NATO countries -- Norway, Italy, Turkey and Luxembourg -- expect increases in defense budgets next year. All of the largest contributors to the NATO defense expect reductions or flat military budgets. In the past, NATO has argued consistently that it need 3 percent real growth each year.

Carlucci, who urged NATO to pursue improvements "vigorously," later said, "I'm not going to make the claim that there will be any large-scale improvement in capability given the constraints we all face."

The Pentagon chief has suggested that NATO use innovative approaches to stretching declining defense funds. But NATO continues to be thwarted by its internal schisms.

While most NATO leaders agree that economic problems will preclude increasing troop levels, and in fact are likely to lead to reductions, they have been unable to agree on more-efficient ways of distributing the manpower. Some of the force structure has changed little since World War II.

The disagreements are even more difficult in discussions on ways of approaching conventional arms control, considered far more complex and tedious than nuclear arms control negotiations because of the combination of geographical considerations, numerous types of equipment and serious verification questions.

NATO officials say they hope that the political atmosphere created by the INF treaty will help smooth the way for a new conventional arms control forum.

The United States, after much internal haggling, two weeks ago offered NATO a new negotiating approach on conventional arms control. Administration officials say the U.S. approach would require Warsaw Pact nations to cut the number of tanks and artillery weapons about 50 percent to numbers equivalant to that of NATO forces. Those proposals would not be formally submitted to NATO until well into next year, however.