A major cutback in commercial bank lending to Third World countries is now underway, threatening both the borrowing countries and the Western world with a new global economic crisis, a number of leading bankers said here today.

Geoffrey Bell, executive vice president of Schroder International Ltd. and John Heimann, cochairman of Warburg Paribas Becker, estimated that there will be a gap of $45 billion to $50 billion in the borrowing needs of Third World countries in 1983 that commercial banks will either be unable or unwilling to supply.

They and others suggested that unless both the World Bank and International Monetary Fund step into the breach, there will be a further contraction of economic activity in the Third World, and that a reduction in its ability to buy goods from the industrial world would extend the recession in Europe, Asia and the United States. At the moment, the IMF and World Bank together are scheduled to lend these countries less than $14 billion.

These pessimistic comments were made during the first day of a two-day meeting on debt and related interest rate problems sponsored jointly by the Global Independence Center here and the Group of Thirty, a private organization of international experts studying how the economic system works.

In a luncheon address, Heimann said that "there is a solvency problem for the less developed countries in the short term. If there is a gap of $45 billion to $50 billion, the question is: Where will the money come from, and, if it is not supplied, will it cause the failure of the international banking system?"

Heimann said the situation was not analagous to the deflationary problems of the 1930s. "It's a situation caused by bad bankers who made bad loans to these countries," he said. In the past few years, the major international banks have been boosting their loans to the LDCs at a 25 percent annual rate.

But Heimann said the wrong solution would be for the bankers to try to reschedule their loans on a short term basis, even though it may be tempting to do so. He was especially critical of those smaller banks that have been participating in international loans but that are now attempting to pull out.

Bell said that the resources of the IMF should be boosted by a minimum increase of 50 percent in its quotas -- the deposits by member countries -- and beyond that, an emergency fund of at least $20 billion should be created for use before new quotas come into effect.

But he, C. Fred Bergsten, director of the Washington Institute of International Economics, and other speakers emphasized the need for the World Bank to join with the IMF in attempting to repair the debt structure of the poor nations, which now stands at $500 billion.

"We are running out of time," warned Herman C. van der Wyck, managing director of SG Warburg & Co. Ltd. He argued that if the poor countries do not have access to enough new funds, there would be a growth of protectionism "that could be suicidal for the Western world."

In another panel discussion, Kurt Richebaecher, a consultant and former economist for the Dresdner Bank of West Germany, predicted that the U.S. dollar would decline sharply as a result of an impending U.S. current account deficit--that is, a deficit in both trade and services. Richebaecher said that the United States has been following an "absurd monetarist policy" that had seriously depressed the European economy.

Bergsten again called for capital controls on the outflow of Japanese yen as an interim measure to bolster the Japanese currency, which has been sharply depreciating against the dollar. He said unless steps were taken to bring the yen into better equilibrium with the dollar, Japan's trade balance "would soar" as the cheaper value of the yen permitted relatively lower prices for its exports. "And if you think you've seen protectionism," he added, "you ain't seen nothing yet."