A breakdown between Third World nations and their creditor banks could result if a number of banks follow the example set by Bank of Boston Corp. and write off part of their Third World loans, according to federal officials and banking industry sources.

Some banking experts voiced fears yesterday that such a development could provoke a financial crisis in which developing countries would default on their debts. These experts fretted that debtors would perceive bank writeoffs as an invitation to pay less than they owed.

But longtime critics of the banks hailed the move by Bank of Boston, announced Monday, to write off $200 million of its $1 billion exposure in the Third World. "This is acknowledging reality," said Rep. Charles Schumer (D-N.Y.), who advocates scrapping the traditional approach to the debt problem and forgiving a portion of the developing countries' financial burden.

"It's the wave of the future," Schumer added, saying other banks will be forced to follow Bank of Boston.

Bank of Boston's action comes at a time when new doubts have arisen about the ability and willingness of big Latin American debtors, especially Argentina and Brazil, to service their multibillion-dollar debts. Both countries are suffering from severe economic problems and deep political divisions.

Most bank officials contacted yesterday for response to Bank of Boston's writeoff were guarded and noncommittal. They said they were studying the move, which goes substantially beyond the action taken last May by Citicorp and other big lenders to boost reserves against possible Third World loan losses.

Adding to loan-loss reserves, as Citicorp did, is a first step that banks take when they believe a borrower may be unable to repay a debt. It causes a decrease in profits, but leaves intact the bank's capital -- its financial cushion. Writing off a loan entails a reduction in capital, and some bankers yesterday expressed anger that Bank of Boston would go as far as it did.

"They're saying their claims aren't worth anything," fumed one top official at a large New York bank. "They're saying, 'While there's a great deal of uncertainty, we're not even going to try to turn these claims into something of value.' "

Opinion was divided on whether other banks will feel obliged to take their own writeoffs. Some analysts noted that Bank of Boston's Third World loan problems may be unique because the bank lent large sums to private companies and individuals in Latin America, rather than just to governments. But others, including some bankers, said that Bank of Boston's move puts pressure on banks to follow suit. The most reluctant banks, they said, would be the biggest "money-center" banks, such as Citicorp and Chase Manhattan, which could see much of their capital wiped out if they wrote off a big percentage of their Third World loan exposure. Regional banks like Bank of Boston have less of their capital so exposed.

Experts generally agreed that if a wave of writeoffs occurs, it could pose a serious threat to the approach that western governments, banks and developing countries have taken to the debt crisis since it erupted in 1982. In most cases, a troubled debtor nation agrees to adopt austerity policies and continue paying interest on its loans, in exchange for fresh capital from banks and international lending institutions. The idea is to keep the financial system afloat while the debtor straightens out its economy. But the latest developments raise new doubts about whether the traditional approach can continue.

"There is a risk of a cumulative downward spiral of this process," an executive said at one large midwestern bank. The executive recalled that after the Citicorp action last May, "debtor countries began to act increasingly as if their debts were worth less than 100 cents on the dollar. So I think this will produce a further diminishing in the willingness of the debtor countries to pay."

Another banking industry source agreed: "If this action {by Bank of Boston} is followed by other banks, it will make it that much more difficult for the developing countries to get new funds."

Federal bank regulators said they share some of the same worries. But they pointed out that so far, the most important debtor nations have refrained from going all the way to outright default, which suggests that both sides -- banks and debtors -- see a continuation of the current system as in their best interest.

{Meanwhile, Western commercial banks are giving Brazil $3 billion worth of new financing in exchange for assurances that the developing world's most indebted country will scrap a 10-month-old moratorium on interest payments, the Associated Press reported. Representatives of the Brazilian government and the creditor banks, meeting in New York, said in a press statement that the package was part of a broader $4.5 billion refinancing arrangement and was expected to become effective in a few days.}