Many of the world's most important bankers believe that the growing inability of many countries to pay off their debts on time is a threat to individual banks and the international banking system.

Furthermore, according to a survey of major multinational banks, this concern is heightened by parallel difficulties among private companies in the United States, Europe and elsewhere. Together these problems could pose a "serious challenge" to the banking system, concluded the report by the Group of 30, a private assemblage of economists and financiers.

Although international defaults--when countries just stop paying off loans--occur infrequently, in recent years many countries have had to go back to their bankers to renegotiate the terms of their loans. Early this year, strife-torn Poland got easier terms from more than 400 multinational lenders. Nicaragua, Zaire, and Sudan, among others, also have had to change the terms of their loans.

The banks eventually get their money in such cases, but they often suffer losses because of deferred interest payments, stretched-out repayments of principal and lowered interest rates. Rejuggling the terms of a loan because of a county's inability to meet the original terms is called "rescheduling."

According to a survey of 111 major world banks by the Group of 30, more than half of the banks that replied are "unsure whether existing rescheduling arrangements would be able to tide over the system in the event of more frequent and larger reschedulings." Most reschedulings are done on an ad hoc basis, and the negotiation process between the banks and the debtor country is improvised.

But none of the banks suggested any changes in procedures.

Although the study primarily concerned international lending risks, the bankers cited the "increasing pressure on corporate balance sheets and a growing number of bankruptcies in Western countries" as adding to the risks the international banking system faces. The list of U.S. companies with debt problems has grown sharply in the last few months, for example.

The study notes that bankers as a group tend to be excessively cautious. " 'Concern' on a banker's part is not, therfore, always synonymous with 'worry'," it says.

Despite the bankers' growing concern about international lending risks, 75 percent of them think their international lending and earnings will continue to grow faster than their other businesses.

The Group of 30 is an ad hoc group of economists and financiers from the public and private sectors in a number of countries who meet several times of year to examine problems in the international economic system. The group was established in late 1978 with an initial grant from the Rockefeller Foundation.

Among the members are Anthony Solomon, president of the New York Federal Reserve Bank; Henry Wallich, a member of the Federal Reserve Board; and John Heimann, former comptroller of the currency. Heimann, who is co-chairman of the international banking risk study group, is chairman of the executive committee of the investment firm Warburg, Paribas, Becker.