The CIA will be unable to continue its covert support of anti-government guerrillas in Nicaragua after Oct. 1 if the House sticks by the opposition it expressed Thursday night in a 228-to-195 vote, House leaders said yesterday.

This little-understood legislative and legal situation gives added significance to the House vote, according to Intelligence Committee Chairman Edward P. Boland (D-Mass.), one of the principal sponsors of the bill cutting off U.S. aid to the "secret war."

Boland's disclosure suggested an earlier-than-expected onset of the "giant headache down the road a ways" that was forecast Tuesday night by President Reagan if opponents of the Nicaraguan covert rebel operation have their way.

Reagan, through a spokesman, expressed disappointment at Thursday night's House vote, but said it "represents but one stage in the continuing congressional consideration of this issue." The statement said the administration will continue to pursue its current policies in Central America.

Reagan, in an interview taped for broadcast tonight on the McLaughlin Group program on WRC-TV, said he was "not at all dismayed" by the House vote. He said it was "so close" and insisted that "there is a solid consensus on our side. Granted, it isn't a majority, because the leadership--the Democratic leadership--is obviously on that other side. For what reason, I don't know."

Because it seemed unlikely to become law, the Boland-Zablocki bill passed Thursday night after an intense and lengthy debate was interpreted mainly as a political signal of House and Democratic Party opposition to the "secret war" against Nicaragua's Sandinista government.

Technically, the bill is an amendment to the fiscal 1983 Intelligence Authorization Act, which has been passed by both chambers and signed into law, so if the Senate takes no action, the Boland-Zablocki measure will not take effect.

However, opponents of the CIA covert operation will have another opportunity to express their opposition when the House considers the intelligence authorization bill for fiscal 1984, which begins Oct. 1.

In a report filed May 16, the Democratic majority of the House Intelligence Committee recommended deleting all funds for the Nicaraguan covert operation from this bill. The precedent, reasoning and vote count of Thursday night suggest that the House would back up the committee when the measure reaches the floor.

House refusal to authorize the funds would give that chamber real leverage through the power of the purse, and would mean that the administration would be in a bind after Oct. 1. Even the continued backing of the White House and the approval of the Senate will not permit the spending of public money without House approval in the new fiscal year, according to legislative and legal experts.

The CIA and the White House are allowed to spend some funds in emergency situations without specific congressional authorization, but it is unclear whether these legally could be used for a purpose explicitly disapproved by the House. Even if this could be done, Boland said yesterday, "whatever funds they could find would not be sufficient to carry this Nicaraguan covert operation on."

The amounts of money involved in backing the "secret war" have been considered classified information, but Rep. Henry J. Hyde (R-Ill.), a supporter of the CIA operation, inadvertently made them public toward the end of Thursday night's House debate.

Noting that the Boland-Zablocki bill calls for $30 million to be spent in fiscal 1983 and $50 million in fiscal 1984 for overt, or open, interdiction of illicit arms in Central America, Hyde said: "Those are the sums that the administration wanted for covert interdiction."

Asked about this yesterday, Hyde said this was "a supposition" on his part rather than a statement of facts, but other sources said Hyde had "spilled the beans."

The CIA, asked if it could continue to support the "secret war" after Oct. 1 in the absence of an authorization by the House, replied through a spokesman: "We will only spend what is in our budget."

On the political level, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said the House vote "will awaken the president, who is flexing his muscles on a macho trip" in Central America.

O'Neill said that despite words from the White House that policy will not be changed, Reagan "is going to have to think seriously" about the meaning of the House vote. He predicted that in the end Reagan will change his Central American policy in the face of congressional and public opposition, and he cited earlier turnabouts he attributed to the president on Social Security, public works, tax issues and other matters before Congress.

"I'm expecting that he'll change his views on Central America and take advantage of the opportunities that are out there for negotiations," O'Neill said.

Saying that Latin American countries strongly favor a U.S. policy based on negotiations rather than military force and that both Nicaragua and Cuba "have extended the hand of peace down there," O'Neill said that the United States at present is "the only one in the hemisphere that is out of step."

O'Neill rejected the view that the solid Democratic majority on the Boland-Zablocki bill reflected partisanship by the Democrats. "If the White House had not worked so hard on the Republicans, arguing that they had to back the president, we would have had 100 Republicans on our side," he said.

Across Capitol Hill, Sen. Edward M. Kennedy (D-Mass.) said the House vote "sent a clear and unequivocal message to this administration that it is time to end the dangerous escalation of covert warfare in Central America."

Kennedy said he will seek to hold the Boland-Zablocki bill at the Senate desk when it reaches that body, rather than let it be referred to a committee, "so that it can be called up before the full Senate for a vote . . . . I will insist that every senator stand up and be counted on the vital question of war and peace in Central America."

The administration, in a response to expressions of concern in the Senate about its Central American policies, has agreed to provide regular briefings every two weeks for Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) and Minority Leader Robert C. Byrd (D-W.Va.), according to Senate sources.

The initial briefing, scheduled for next Wednesday, will be by Deputy Secretary of State Kenneth Dam, according to the sources.

Baker had suggested regular briefings during a closed-door Senate meeting last Wednesday with Secretary of State George P. Shultz and presidential Assistant national security affairs adviser William P. Clark, nothing noting that the Senate leaders had been kept informed in this way by the Carter administration during the long-running Iran hostage episode.

O'Neill had joined Byrd Thursday in a letter to Reagan citing the lack of advance congressional knowledge of the recently ordered large-scale military exercises in Central America and asking for improved consultation with Congress. As of yesterday afternoon, O'Neill said he had received no response.