What Rudyard Kipling called "the great game" of espionage has once again intruded on this forum for world peace. The debate ranges from how the jobs in the U.N. library are divided up to worries about where the latest superpower dispute on reducing the Soviet mission will go and how much the United Nations will suffer.

The Reagan administration set it all off with an unprecedented March 7 order requiring the Soviet, Ukrainian and Byelorussian missions to pare their staffs, which total 275, to 170 by April 1, 1988.

The United States justified the order on grounds that the size of the missions is "a threat to national security." The Soviet Union warned that the move could "do direct damage" to U.S.-Soviet relations. Officials of Secretary General Javier Perez de Cuellar's secretariat regard the action as a legal breach of the procedures specified in the 1947 headquarters agreement between the United States and the U.N.

Delegates of many other member nations, while acknowledging the United States has a justifiable complaint, say that Washington acted in a high-handed manner that ultimately could do more harm than good because it threatens to further erode the U.N.'s limited effectiveness by embroiling it in a new East-West controversy.

But one who has no doubts about the U.S. order is Arkady Shevchenko, who before defecting to the United States in 1978 was the highest-ranking Soviet citizen in the secretariat. In his five years as undersecretary general for political and security council affairs, he was a key intelligence resource for Moscow and, during the latter part of his tenure, a double agent working for the United States.

In his 1985 book, "Breaking with Moscow," Shevchenko asserted, "It is probably no exaggeration to count over half of the more than 700 Soviets in New York City as . . . full-time spies."

Although his count was higher than earlier estimates, Shevchenko's charges were an old story to diplomats. They consider it inevitable that the U.N.'s commingling of nationals from almost every country will tempt member governments to seek to tap its repository of diplomatic secrets and to use it as a base for intelligence gathering.

The Soviets and their East European allies are the most blatant and aggressive, neutral observers say. As a result, one of the constants of the U.N. scene has been the hide-and-seek match that pits them against the FBI and periodically comes to public view with the arrest or deportation of East Bloc diplomats or journalists.

"It's a fact of life, and there is nothing we can do about it except to make the problem more manageable," said Shevchenko, now an American citizen, in a telephone interview.

"Either we take the course adopted by the administration and cut down their numbers, or we must greatly increase the number of FBI agents watching them," he said. "But that second course would cost far more money than the Congress and the American taxpayers are willing to spend. I know that I don't want to take money out of my pocket to pay for watching the Soviets."

Four miles south of the U.N.'s East River complex, the same arguments can be heard in the lower Manhattan federal building skyscraper that houses the 1,135 agents of the FBI's New York field office.

"The Soviet presence in New York is the biggest surveillance problem that we have because there are more Soviets here than anywhere else," said John L. Hogan, assistant FBI director for New York.

"In Washington, they have a little over 500 people. In New York, there are close to 900, including their U.N. mission, Soviet members of the secretariat, people on temporary duty at the U.N., journalists from the Soviet news agencies, members of their commercial companies and working wives of Soviet officials."

Neither Hogan nor James M. Fox, the special agent in charge of New York counterintelligence, will say how many agents are assigned to keeping tabs on the Soviets. But they note that counterintelligence is "the No. 1 priority" of the New York office, and an entire floor of the huge building is filled with agents working at that job.

"Since we don't have anywhere near enough agents to go one-on-one, we have to be very selective about who we target," Hogan said. For that reason, the bureau strongly supports the administration's order and any other moves that might cut the number of Soviets here.

According to Hogan, the Soviets' biggest espionage effort here is directed at the research and development efforts of American business.

"Since we're such an open society, 80 percent of their activities can be overt," he said. "With all the industrial and high-tech trade shows in New York, they literally can make their rounds with a shopping basket."

The FBI also considers New York and the U.N. as a major center for Soviet efforts to recruit agents -- whether Americans seeking to make money or foreign nationals like Arne Treholt, a high-ranking Norwegian diplomat convicted last year for supplying the Soviets in 1980 with information on Washington's policy toward Afghanistan while working at the U.N.

In addition, Fox said, the Soviets use their citizens in the secretariat to obtain information and to gain control of "strategic choke points where they can manipulate U.N. activities to their advantage."

In that respect, he cited not only Shevchenko's political department, which always has been headed by a Soviet, but also the secretary general's executive office and such less obvious posts as the U.N. research library and a news service geared to the Third World.

"They may not sound very important, but they have a lot of uses for the Soviets," Fox said. "If you control these offices, you can slant the information that comes out of them to reflect Soviet positions, and you also can use them to evade normal travel restrictions on bloc personnel by sending them on missions that ostensibly are U.N. business."

Aides to Perez de Cuellar, while unwilling to engage in a public fight with the FBI, say that these charges are vastly exaggerated. Earlier this week, the secretariat issued a public rebuttal to a report by the conservative Heritage Foundation charging that the U.N. library is Soviet-controlled.

It noted that of the 64 professional posts in the library, eight are held by Soviet nationals and 23 by U.S. citizens. It also said that of 35 trips made by library personnel between 1982 and 1985, 15 were by Americans and only two by Soviets.

More importantly, U.N. officials added, successive secretaries general have evolved informal methods over the years for working around secretariat personnel whose loyalties are suspect.

The political department, which Shevchenko headed, is supposed to be the secretariat's chief political arm. In practice, though, the U.N.'s most sensitive political work-dispute settlement efforts and peace-keeping forces have been handled, respectively, by two other undersecretaries of unquestioned loyalty to the U.N. -- Diego Cordovez and, until his recent retirement, Brian Urquhart.

In addition, U.N. officials point out, the U.S. order, while hitting at the Soviet delegations, will have no effect on the Soviet Union's ability, as a Security Council member, to continue controlling a number of secretariat positions.

That is why secretariat officials and sympathetic delegates from other countries believe that the principal result of the U.S. move will be an acrimonious Soviet counterattack that will further damage the U.N.'s stature in U.S. public opinion but that will not signficantly affect Soviet espionage.

Or, as Shevchenko put it:

"The one thing you can be sure of is that if someone raises the old cry of retaliating by taking the U.N. out of New York, the Soviets would never allow that to happen. Even if their presence is reduced, New York will still be a very important intelligence base for them. They will never willingly give it up."