LONDON -- An anticipated U.S.-Soviet agreement to eliminate intermediate-range nuclear weapons from Europe has focused new attention on an old question: how serious is the purported Soviet advantage in conventional weapons and what cuts are fair in the massive firepower and millions of soldiers facing each other across the East-West divide?

One indication that the argument has entered a controversial phase is the decision by London's prestigious International Institute for Strategic Studies to alter radically the format of its own assessment of the conventional balance in Europe, released today.

This year, in an abrupt departure from the past, the institute has gone out of its way to indicate that the statistics, taken by themselves, do not necessarily prove that the Soviet-led forces would prevail in a conventional war. The newly published 1987-88 edition of the institute's "The Military Balance" notes that its figures have been "much quoted, but also at times misrepresented," and that "political leaders, East and West, sometimes use assessments of the 'balance' selectively to rally public support for their foreign and defense policies."

Prime Minister Margaret Thatcher has argued that nuclear cuts might some day tempt the Soviets to use their conventional superiority in Europe to launch a surprise attack. She and other western leaders made their approval of an agreement on intermediate-range nuclear forces contingent on increased emphasis being given to negotiations for cuts in conventional weapons.

While Europeans seem satisfied with this agenda, some U.S. congressional opponents of a treaty limiting medium-range missiles see the existing conventional threat as a justification for delaying ratification of the nuclear accord. Until the Soviet Union and its Eastern European allies actually reduce their superior conventional force levels, these critics argue, the West cannot afford to do without the deterrent power of nuclear weapons.

There is little doubt that the Warsaw Pact is numerically superior to NATO in manpower and virtually every category of conventional land-based weaponry deployed in Central Europe. Yet, as both the debate over medium-range missiles and new East-West negotiations on the conventional balance gain momentum, questions are being raised about what these numbers mean in terms of who would be likely to win a nonnuclear European war.

Since the early 1960s, "The Military Balance" has been widely considered the most authoritative nonclassified accounting of military personnel and hardware around the world on a country-by-country basis. Statistics from its yearly chart comparing Warsaw Pact and NATO force and equipment ratios appear in newspapers, and politicians quote from it to each other and their constituents.

In the past, the institute preferred to allow the figures to speak for themselves. A dry essay accompanying its force-ratio chart traditionally has pointed out only that military aggression would be a "highly risky" endeavor for either side.

Yet the institute now takes the position that its depiction of massive Warsaw Pact superiority has been misconstrued by many in the West, particularly on the right of the political spectrum, as proof of the need for more defense spending and less arms control. Last year the Warsaw Pact had an advantage of 2.29 to 1 in main battle tanks over NATO, nearly 7 to 1 in surface-to-air missiles, and nearly 1.5 to 1 in troop deployments.

The new conventional balance chart no longer includes what had been its most often-quoted listing -- the ratio of Warsaw Pact to NATO forces, computed from the individual statistics. Readers can still easily tabulate the ratio of numerical superiority on their own. But the institute has decided it does not want its name attached to stark, simplistic ratios.

Most significantly, the accompanying essay on the East-West conventional balance in Europe offers a new analysis of the many methods of assessing superiority beyond mere numbers. Quality of equipment, mobilization and resupply time and morale all help determine who might possess the military advantage.

"No single approach is likely to prove useful for all purposes. Indeed, it is a misnomer to speak of a single, overall 'balance,' " the essay notes before going on to look at different methods that can be used for assessing balance, and their limitations.

"We've started to spell out the 999 ways in which pure bean counting doesn't give a sensible answer," said John Cross, a former Defense Ministry analyst and now the official who oversees compilation of "The Military Balance."

"If the object is to say 'who wins,' the answer is 5,000 pages of computer printout, or the terse phrase, 'Well, it depends,' " Cross said. "Both are equally true."

Cross emphasized that "The Military Balance" was not abandoning its past conclusion that military aggression would be "highly risky" for either side, with no assurance of victory. But, he said, "it seemed to us not a particularly meaningful statement, although it was being interpreted as if it was."

The statement, Cross said, had been "quoted to me 100 times by the Russians," who he said also use "The Military Balance" selectively for their own purposes. "It's not untrue," he said, "but what it hides is that it would be equally true within a wide range of force balances. The question is, at what imbalance would it seem to be risky?"

Measuring quality against quantity complicates any assessment of the East-West military balance. Some argue, for example, that the substantial Warsaw Pact advantage in number of battle tanks is offset by the aged condition of many of the tanks and NATO's sophisticated antitank defenses. Others, however, counter that the Soviets are modernizing their tank fleet, and are quickly catching up with the West in antitank technology.

The sudden plunge of the rigorously apolitical study into such subjective debate reflects how urgent its authors feel that debate has become.

"This is going to be a political-media debate, and a high-profile topic over the next few years," Cross said. "It is going to be based on IISS data, because its the only one available."

While both NATO and the Warsaw Pact agree that stability in conventional forces should now be a priority, past experience gives little cause for optimism. During 13 years of talks on Mutual and Balanced Force Reductions that began in 1974 in Vienna, the two alliances have failed to agree even on the size of each other's forces in Europe, let alone how to reduce them significantly.

In the past year, they have decided to scrap those talks altogether and to open a new set of talks, probably to begin next spring. Although the negotiations will be held under the umbrella of the 35-nation Conference on Security and Cooperation in Europe, the Helsinki accords organization, participants will still be limited to the 16 members of NATO and the seven Warsaw Pact states who took part in the Mutual and Balanced Force Reduction negotiations.

The principal difference will be in geographical scope, with the new talks covering all of Europe "from the Atlantic to the Urals," rather than just the Central European zone covered in the earlier talks.

While moving toward the new negotiations, NATO has continued efforts to redress some of the existing conventional imbalances, based on its Conventional Defense Improvement program initiated in May 1985. But such plans may require heavy new defense expenditures, particularly from NATO's European members.

In a speech last month to the Royal United Services Institute here, NATO Secretary General Lord Carrington warned of the dangers of "passive disarmament," should Western Europe not recognize that an INF agreement "will inevitably highlight the conventional element in deterrence and the imbalance which currently exists."

Yet despite such warnings, most NATO members are decreasing, rather than increasing, their military budgets.