A three-judge federal appeals panel yesterday declared unconstitutional the congressional veto, used increasingly in recent years by the House and Senate in an attempt to reverse decisions by federal regulators and executive branch agencies.

The decision, in a natural gas pricing case, could affect provisions in more than 200 federal statutes--ranging from foreign policy to the environment to federal election law to D.C. home rule--which allow Congress to strike down executive actions on its own without further reference to the president.

Some legislative veto provisions allow one house of Congress to overturn an agency action, some require both, but in neither case must Congress send its resolution of disapproval to the president for his signature. Defenders in the longstanding and often bitter controversy over its use say it is the elected legislature's most effective means of controlling the government bureaucracy.

Congress is debating the veto as part of a wide-ranging regulatory reform bill on which both houses are expected to act this year.

The unanimous ruling yesterday by a panel of the U.S. Circuit Court of Appeals here was the broadest decision yet in this dispute over the balance of power among the three branches of the federal government. The U.S. Supreme Court already has agreed to review a narrower case striking down an immigration law provision permitting one house of Congress to override a decision by the attorney general in a deportation case.

Yesterday's decision can be appealed to the full appeals court or to the Supreme Court.

The decision held that legislative vetoes violate the fundamental allocation of power set out in the Constitution and allow Congress to intrude on executive authority.

The opinion, written by Judge Malcolm R. Wilkey, said that if Congress wants to tell the executive branch how to act or what to do the Constitution provides only one way: by a vote of both houses of Congress which must then be sent to the president for his signature or his veto.

"We are aware that our decision today may have far-reaching effects on the operation of national government," Wilkey said in a 104-page opinion. But he said the panel had no choice.

"The genius of our Constitution, its adaptability to changes in the nature of American society, depends ultimately on the steadfastness with which its basic principles and requirements are observed," Wilkey wrote. He was joined in his decision by Senior Judge David L. Bazelon and appeals court Judge Harry T. Edwards.

The panel's decision came in a case involving the Natural Gas Policy Act of 1978, in which Congress relaxed federal price controls on natural gas. Congress decreed in the act that industrial gas users should bear more of the cost of deregulation than residential users. But when the Federal Energy Regulatory Commission then issued a regulation in May, 1980, to carry out this provision, the House vetoed it, thereby threatening to shift more of the cost back to householders.

The constitutionality of the veto provision in the act then was challenged in the appeals court by the Consumer Energy Council of America, the Consumer Federation of America and Ralph Nader's Public Citizen litigation group. They charged that the unilateral veto by Congress deprived the president of his veto power, infringed on the doctrine of separation of powers and violated the constitutional principle that both the House and Senate must pass laws.

Litigation group lawyer Alan B. Morrison said that the veto in the natural gas case destroyed an essential element of the legislative compromises that led to passage of the gas act--the promise that price increases would be shifted away from consumers.

Both houses of Congress, which entered the case with the agency, argued that the veto did not change the law--and thus require the full procedure for lawmaking. Rather, they argued that legislative rejection of the new regulation merely preserved the status quo. Such limited exercises amount to a legitimate sharing of power that Congress originally gave these agencies, they contended.

In defending legislative vetoes, Congress has taken a flexible view of the separation of powers, contending that the branches are not "hermetically" sealed off from each other.

"Congress sees its legislation distorted and turned around through rules and regulations," Rep. Elliott H. Levitas (D-Ga.) said yesterday. Levitas is a main sponsor of an amendment to the regulatory reform bill that would allow legislative veto without the president's signature.

Wilkey, however, said that the legislative veto was a device that enables Congress "to expand its role from one of oversight, with an eye to legislative revision, to one of shared administration. This overall increase in congressional power contravenes the fundamental purpose of the separation of powers doctrine."