The world's leading industrial nations agreed yesterday to a major strengthening of the International Monetary Fund's resources to fend off potential default by Third World countries on their international debts.

At the same time, in a significant policy reversal, the western nations that make up the Group of 10 agreed to stress "renewed and sustained" economic growth to counter a global recession that is becoming the worst since the Great Depression. Until now, the policy focus of the major nations and the international lending organizations such as the IMF has been to fight inflation.

The finance ministers of the 10 wealthy nations, meeting in Paris, took two steps to enlarge the resources of the IMF, the Washington-based arm of the World Bank that provides loans to less developed nations. They created a new $19 billion emergency fund, available to any IMF member with problems that "pose a threat to the stability of the international monetary system." An existing $7 billion fund of the Group of 10, known as the General Agreement to Borrow, or GAB, has been available only to the rich donor countries themselves.

They also promised to support a "substantial" increase in the normal resources of the IMF--the quotas, or deposits, put up by the member countries--at a meeting of the IMF policy board in Washington Feb. 10 and 11.

The larger quotas--which have to be approved by national legislatures--are technically not due to come into effect until 1985, and only a portion of the $32 billion increase would be in hard currencies desired by borrowing countries. But a Group of 10 communique said that it is desirable that the new quotas come into effect by the end of 1983.

Even if larger quotas do not become effective until 1985, the knowledge that the IMF reserves will be expanded and replenished is expected to encourage commercial banks to keep up their lending programs.

The pledge that the rich countries would undertake steps by midyear to "ensure renewed and sustained growth" is considered equally important, a recognition that larger loan resources alone will not move the world out of the recession. Treasury Undersecretary Beryl Sprinkel argued in Paris that those countries, including the United States, that have made major progress in reducing inflation should turn attention now to stimulating economic growth.

No details were made available on the exact steps the leading nations could take to stimulate growth. Conference chairman Jacques Delors, France's finance minister, cautioned that "we are not magicians" who can create an economic recovery overnight.

But the new emphasis on the urgency of economic growth and the reduction of unemployment amounts to an about-face on international economic policy. Secretary of State George P. Shultz recently warned that without new global growth, Third World countries could never earn enough money to pay off their international debts.

The two major ingredients of a boosted IMF money package--the emergency GAB fund and larger IMF quotas--have been under intense negotiation since last summer, when Mexico's debt crisis stunned the international financial world. Informed sources here and in Paris said yesterday that the likely boost in quotas would be from the equivalent of $67.2 billion to $99 billion, or 47 percent.

There have been fears that IMF funds would soon be exhausted, leading to a collapse of the international financial system, including defaults on developing country debts that might, as a consequence, bankrupt some major banks.

The enlarged and liberalized emergency fund will be more quickly available for depressed economies, which have been forced to ask for postponement of both interest and principal payments on their debt. All told, Third World countries have external debts of about $500 billion, of which 60 percent is owed to commercial banks.

Although the Reagan administration has agreed to a progressively more liberal stance on IMF funding as the potential for defaults--especially in South America--has worsened, it has resisted pressures from the borrowing nations for a doubling of IMF quotas.

Even a 47 percent increase, White House officials fear, will be difficult to get through Congress, even though U.S. contributions are not charged against the budget.

Many members of Congress are likely to challenge larger IMF quotas on the grounds that they will be used to bail out commercial banks. Others may resist approval because of the general pressure to demonstrate austerity in a big budget-deficit year.

The Group of 10 was originally set up in 1962. The participating countries are the United States, Canada, Belgium, France, Great Britain, Japan, Italy, the Netherlands, West Germany and Sweden. It was announced yesterday that Switzerland will soon join the group, making it the Group of 11.

Separately, negotiations are under way with Saudi Arabia, not a member, to add about $4 billion to the emergency package. If the Saudis advance that sum, and the IMF policy board approves a 47 percent quota increase at the meeting here in February, the IMF will be getting about $30 billion more than it now has in lendable currencies to meet what is expected to be a tremendous demand for loans from Third World nations in the next few years.