WASHINGTON (AP) _ Federal banking agencies are ordering the nation's largest banks to more than double reserves set aside for anticipated losses on loans to economically troubled Argentina.

The order, expected to depress industry earnings in the fourth quarter, requires banks to hold reserves of at least 35 percent of the loan amount, up from 15 percent, said a Federal Deposit Insurance Corp. official who spoke Wednesday on condition of anonymity.

It is unclear how the decision will affect individual banks.

Some likely already have loss reserves large enough to cover 35 percent of their loans to Argentina. But others, such as Citicorp and BankAmerica Corp. of San Francisco, have proportionately smaller loss reserves for Third World debt than competitors.

Jim Mitchell, a spokesman for BankAmerica Corp., said the bank was aware of the new reserve requirement but declined to say what the effect would be on his institution.

''A discussion of our LDC (less developed country) reserve coverage and related issues will be a routine part of our (fourth-quarter) earnings announcement, which we expect to make next week,'' he said.

BankAmerica Corp.'s stock was unchanged Wednesday at $27.50 on the New York Stock Exchange.

Prices fell for the shares of several other large banks.

Chase Manhattan closed at $31.50, down 37 1/2 cents; Chemical Bank, $30.12 1/2 , down 12 1/2 cents; Citicorp, $27, down 25 cents; and Manufacturers Hanover Trust, $32.25, down $1.

In the third quarter, the nation's 12,800 commercial banks collectively recorded a $744 million loss, the first in two years. The industry was pushed into the red by $5.3 billion in Third World debt losses at five large banks.

The decision on Argentina loans likely will produce another disappointing report for the fourth quarter. It stems from a recommendation from the Interagency Country Review Committee, which coordinates Third World lending matters among banking agencies, including the FDIC, the Federal Reserve Board and the Comptroller of the Currency.

The committee customarily does not announce its decisions.

Argentina, which owed $6.2 billion to U.S. banks at the end of September, is Latin America's fourth-largest debtor to U.S. financial institutions. The three largest debtors are: Brazil, which owes them $19.1 billion; Mexico, $15.7 billion; and Venezuela, $6.9 billion.

Mexico has scrupulously kept up its payments, but both Brazil and Argentina are in arrears.

Argentina's economy has deteriorated sharply recently, beset by hyperinflation. Its currency, the austral, has lost 99 percent of its value over the past year.

On Monday, prices of the leading stocks on the Buenos Aires Stock Exchange plunged 53 percent after the exchange was closed for more than a week.