In a closely guarded decision that surprised B-1 proponents and opponents alike, President Carter said yesterday he is halting further production of the super-expensive supersonic bomber, and plans to deploy the still-experimental cruise missile in its place.

While only a half-dozen or so people knew what his choice would be even 12 hours before it was announced, the decision is sweeping in its effect. It will affect relations with the Soviet Union in general, may change the course of the Strategic Arms Limitation Talks in particular, and will have an impact on the NATO defense alliance.

It will also alter the defense budget and the futures of the private defense contractors involved. Rockwell International, prime contractor on the B-1, said it will have to lay off about 10,000 workers. Rockwell stock dropped by more than 10 per cent after the Carter announcement.

Carter chose the first few minutes of his 10th formal, televised news conference to make public the decision he has been working intensively on all month. He called it "one of the most difficult decisions that I have made since I have been in office."

An aide said later that was because of its sweeping impact, the judgments

Pentagon sources said production of the cruise missile, a pilotless, winged aircraft that can hit targets with tremendous accuracy over a range potentially greater than 2,000 miles, will now be accelerated. Cruise missiles are still being tested, and until yesterday, the Defense Department planned to have some combat ready by 1980.

The cruise missile has been one of the key unresolved issues in efforts to negotiate a new strategic arms limitation agreement with the Soviet Union.

The Soviets claim it adds a new weapon to the U.S. arsenal and that its range and accuracy make it a strategic weapon that must be included in any new agreement.

A Soviet military official hinted in May that the Soviet Union will build a new generation of its own cruise missiles, as well as new defenses against the weapon, if the American missile moved from what was then its experimental test status into deployment.

Carter announced at his news conference that he will continue the "existing testing and development program now under way on the B-1" to "provide us with the needed technical base in the unlikely event that more cost-effective alternative systems should run into difficulty."

A White House spokesman said, however, that the three planes already in Defense Department hands, and a fourth currently in production, are the only ones that will be built.

The House voted Monday 243 to 178 to spend $1.5 billion to build five B-1s. White House press secretary Jody Powell said he is "not sure it's been completely determined" how much of that money will need to be diverted to pay for speeded-up cruise missile development and other defense spending.

"It wasn't a question of spending the money on the B-1 or putting it in a savings bank," he said.

Carter told his news conference that "during the coming months we will also be able to assess the progress toward agreements on strategic arms limitations in order to determine the need for any additional investments in nuclear weapons delivery systems.

In the meantime," he continued, "we should begin deployment of cruise missiles using air-launched platforms, such as our B-52s modernized as necessary.

"Our triad concept of attaining three basic delivery systems will be continued with submarine-launched ballistic missiles, intercontinental ballistic missiles and a bomber fleet, including cruise missiles as one of its elements. We will continue thereby to have an effective and flexible strategic force whose capability is fully sufficient for our national defense."

Carter said he thinks both the House and Senate will confirm his decision. Powell said he senses that congressional leaders "are receptive to the President's decision in this manner so long as he will proceed, as we said, to maintain the triad and to deploy and effective alternative."

During the past three weeks, Powell said, Carter, Defense Secretary Harold Brown, the National Security Council and the Office of Management and Budget developed a series of options that included varying mixes of cruise missiles and B-1s.

But he was searching, Powell said, for the maximum deterrent for the dollar, and the B-1, the most expensive combat aircraft ever built at an estimated $102 million each, would not provide that, either alone or coupled with the cruise missile.

"He clearly believes the cruise missile is an effective weapons system and is confident of its capabilities," Powell said.

Carter campaigned last year against Air Force hopes to buy 244 of the planes. Last July he told the Democratic National Convention Platform Committee. "The B-1 bomber is an example of a proposed system which should not be funded and would be wasteful of taxpayers dollars."

But Carter said yesterday he tried to arrive at his decision with an open mind, after studing "classified analyses and information about weapons systems which I did not have before . . ."

Brown, whom Carter consulted with dozens of times as he deliberated, is on record favoring a new bomber system, although not necessarily the B-1.

The major factors in his decision, Carter said, included "the recent evolucion of the cruise missile as an effective weapon itself," and "the continued ability to use the B-52 bombers . . . up will into the 1980s, and the belief on my part that our defense capability using the submarine-launched missiles and intercontinental ballistic missiles combined with the B-52 cruise missile combination is adequate."