The chairman of the House Intelligence oversight subcommittee, just returned from a trip to Central America, said yesterday that the Reagan administration is breaking a law that prohibits attempts to overthrow the leftist government of Nicaragua.

His statement is the most authoritative congressional assertion to date that the two-year-old covert CIA operation has broken the legal bounds imposed by Congress.

Rep. Wyche Fowler Jr. (D-Ga.) told a news conference that "under the best of circumstances" the congressional ban, known as the Boland amendment, "is not being fully adhered to."

Fowler said the full committee chairman, Rep. Edward P. Boland (D-Mass.), had assured him that the panel would hold hearings "as soon as possible next week to discuss measures to ensure compliance with the law."

Boland relayed this assurance by telephone from the Far East, where he is traveling.

Earlier this week, Sen. Daniel Patrick Moynihan (D-N.Y.), vice chairman of the Senate Intelligence Committee, and Sen. Patrick J. Leahy (D-Vt.) expressed concern that the administration may be violating the spirit, if not the letter, of the Boland amendment.

That committee is also scheduled to hold a closed-door hearing on the Nicaraguan campaign next Tuesday.

The language in the Boland amendment was first passed in secret by a House-Senate budget conference last August and then sponsored publicly by Boland during the December budget resolution debate.

It forbids the administration from taking any action "for the purpose of overthrowing the government of Nicaragua or provoking a military exchange between Nicaragua and Honduras."

Fowler said that he would introduce legislation next week to set statutory standards for covert actions to make sure that such "extraordinary" activities are essential to national security; that potential benefits outweigh risks, and that covert objectives are consistent with "publicly avowed foreign policy."

The legislation also would give the House and Senate Intelligence oversight committees veto power "to disapprove such operations."

Fowler was careful to state that his conclusion that the administration is violating the Boland amendment was based on "publicly available information and my unclassified discussions with individuals in Washington and Managua."

But as chairman of the oversight subcommittee, Fowler has been fully and regularly briefed every 60 days, he said, on the details of two U.S. covert operations.

Other officials have said their twofold purpose is to interdict the Cuban-Nicaraguan arms supply to insurgents in El Salvador and to provide support to guerrillas fighting inside Nicaragua to topple the Sandinista regime.

Fowler said he would not comment on "any specific evidence relating to my responsibilities on the committee." But he added, "I think that when you see on television every evening the guerrillas openly announcing their objective of overthrowing the Sandinista government. . . . It does not take a strict legal construction by a battery of lawyers to say the spirit and the letter of the Boland amendment are seriously in question."

The House Intelligence Committee staff circulated a memorandum on March 23 instructing committee members on the options they could pursue to oppose covert operations. The options included calling a secret House session to demand a halt to the action, striking funds from the CIA budget and informing the public about the covert action.

State Department officials yesterday expressed puzzlement over a published report that high-ranking members of the department questioned the legality of Nicaraguan operations during White House meetings last week. One official said no one had been able to identify meetings or expressions of this sort during that period of time.

State Department spokesman John Hughes told reporters that senior department officials, including Secretary of State George P. Shultz, "do not believe we are violating the law."

At the White House, deputy press secretary Lyndon K. Allin said, "To the best of our knowledge, there have been no suggestions from senior administration officials that we're in violation of the law."