The Supreme Court, driving home its June 23 decision striking down the "legislative veto," yesterday overturned congressional efforts to block federal regulations concerning used-car sales and the pricing of natural gas.

Acting on the last day of the term, the justices explicitly affirmed lower court rulings setting aside a two-house veto of a Federal Trade Commission used-car rule and a one-house veto of a so-called incremental gas pricing program promulgated by the Federal Energy Regulatory Commission.

The June 23 decision came in a case involving a one-house veto. Yesterday's action made clear its application to two-house vetoes as well.

But it offered no new guidelines on another question that hangs over the nearly 200 laws containing the legislative veto.

That is whether, in each of the cases, Congress would have delegated the authority it did without also including a veto provision.

If the answer is yes, under a Supreme Court standard enunciated in 1976, the underlying law remains on the books. If the answer is no, then the authority that was checked by the legislative veto collapses along with the veto.

This so-called "severability" question was raised in the FERC case, The U.S. Circuit Court of Appeals here held in January, 1982, that Congress would have authorized the incremental pricing scheme as part of the Natural Gas Policy Act of 1978 even without the veto provision. The Supreme Court yesterday accepted that judgment.

Justice Byron R. White dissented from yesterday's decision, in part because he said he was convinced that the veto was an essential part of the House-Senate compromise that led to passage of the 1978 law.

". . . Much of the legislative history suggests that Congress would not have granted the commission unfettered rulemaking authority," White said in a footnote.

But he said the high court's action was "hardly surprising" in light of its June 23 decision in a deportation case involving a Kenyan-born East Indian, Jagdish Rai Chadha. The House of Representatives voted in 1976 to overturn a decision of the attorney general and require Chadha's ouster; the case was then taken to court.

The Supreme Court used it to nullify the legislative veto in a broad-gauged decision affecting laws ranging from arms sales to presidential deployment of troops abroad to the rule-making activity of independent federal agencies.

With White writing a fervent dissent, the court held that, except for certain exceptions spelled out in the Constitution--impeachment cases, for example--all legislative actions must be presented to the president before becoming law.

Yesterday's decision explicitly extended that principle to congressional vetoes of federal rule-making agencies, even though those agencies customarily issue their rules without the president's concurrence.

The used-car rule, promulgated by the FTC in August, 1981, requires dealers to post a window sticker on the vehicles they want to sell, describing various major mechanical defects and setting out any warranties that are being offered.

Under a 1980 law, however, Congress staked out the power to impose a two-house veto on any final trade regulation by the FTC. The Senate rejected the used-car rule on May 18, 1982, and the House followed suit 10 days later.

Consumers Union and Public Citizen Inc. challenged the veto in federal court. The appeals court here held the congressional action unconstitutional last fall, citing its own earlier decision in the one-house veto of the FERC rule.

The FERC rule grew out of Congress' determination in 1978 to provide for the phased deregulation of natural gas prices after a long impasse between the House and Senate. To ease the transition to complete deregulation, the law provided for a two-stage "incremental pricing" scheme, with the second phase to be subject to a veto of either house.

In May, 1980, the FERC adopted its so-called Phase II rules, applicable to industrial users generally. The House vetoed the plan later that month. Consumer groups at first petitioned FERC for reconsideration and, when that failed, moved into the courts.

The U.S. Circuit Court of Appeals struck down this veto last year in a lengthy opinion that examined the legislative history of the 1978 Gas Policy Act in accordance with the statute-by-statute test that now seems to await dozens of other laws.

"We are convinced," the appellate court said in its January, 1982, decision, "that Congress would have enacted both phases of incremental pricing without the veto provision."

The Supreme Court majority affirmed that decision without comment. Justice William H. Rehnquist said he would have given both cases full consideration and set them down for oral argument. Justice Lewis F. Powell Jr. took no part in the deliberations.

As a result, the only pronouncements came from White, who protested that the two cases "illustrate the constitutional myopia" of the June 23 Chadha decision and cast further light on its "destructiveness."

"Congress, with the president's consent, characteristically empowers the agencies to issue regulations," White said. "These regulations have the force of law without the president's concurrence; nor can he veto them if he disagrees with the law that they make."

Now, with the congressional veto invalidated, White declared, the independent agencies, once created, become "a fourth branch of government" not subject to the direct control of either Congress or the executive branch.

The used-car rule is expected to wind up back before the FTC for a vote on when to put it into effect. After that, predicted Alan Morrison, an attorney for Public Citizen Inc., "undoubtedly the car dealers will go into court."

As for the gas pricing rule, FERC will have several weeks to reconsider it, a step that Morrison said "seems to make sense" since the proposal is three years old. In any event, he said, "Congress could come in and deal with this on its own"--by passing an old-fashioned law.