President Reagan will break through the weapon limits of the SALT II treaty Friday when the 131st B52 bomber armed with nuclear-tipped cruise missiles becomes operational, the administration announced yesterday.

The White House called key lawmakers at 5 p.m., and the Defense Department shortly afterward told reporters that Reagan made the decision Tuesday during a meeting with Defense Secretary Caspar W. Weinberger and Secretary of State George P. Shultz.

Pentagon spokesman Robert B. Sims said Reagan had decided that "we couldn't afford to reduce our future deterrent force structure" by dismantling any of three aging Poseidon submarines, each of which carries 16 missiles, to stay within the limits of the unratified SALT II pact negotiated by the Carter administration in 1979 and adhered to by Reagan until now.

Once the 131st B52 armed with cruise missiles flies to Carswell Air Force Base near Fort Worth, Tex., to become operational with the Strategic Air Command 7th Bomb Wing, the treaty limits will be exceeded, the Pentagon said.

Backers of adhering to the limits immediately criticized the president's decision. Sen. Albert Gore Jr. (D-Tenn.), for example, said it "is a mistake in judgment every bit as serious as that which led to the delivery of missiles to the Ayatollah Khomeini [of Iran]. It will cause an uproar in the North Atlantic Treaty Organization alliance at a time our leadership is being questioned not only because of the Khomeini arms deal but because of [the U.S.-Soviet summit at] Reykjavik."

Gore charged that the administration waited until Thanksgiving eve to make its decision known in hopes it would catch little public attention. Sims denied this was the intent, saying the decision was announced shortly after Reagan made it.

"This step reflects the president's May 27 decision that current and future decisions reflecting our strategic forces must be based on overall U.S. military requirements and the threat we face," a Pentagon statement said. In May, Reagan attributed his decision to "direct Soviet noncompliance" with the SALT II treaty.

Some officials connected with the B52 program said yesterday that the White House decision caught them by surprise. The 131st cruise-missile-carrying bomber was initially scheduled to become operational on Dec. 22. It left a paint hangar in San Antonio only a week ago, and was supposed to undergo a month of additional tests.

"The first I heard of it was on the radio going home in my car," said a senior program official. "My understanding is that the bomber still hasn't been fully flight-tested."

Deployment of the bomber signals a formal rejection by the administration of a plea by both houses of Congress last month that the United States "continue voluntary compliance with the central numerical sublimits of the SALT II treaty . . . as long as the Soviet Union complies with such sublimits."

The 131st bomber pushes the United States over a provision of the treaty limiting each side to 1,320 strategic nuclear delivery systems, including land- and sea-based missiles with multiple warheads and cruise-missile-carrying bombers.

Congress said in the defense bill that compliance with this sublimit was "in the national security interests of the United States" after hearing testimony from some arms control and intelligence community experts that a violation would benefit the Soviet Union more than the United States.

The reason, according to the experts, was that the Soviets are poised to deploy thousands more strategic weapons than the United States in the near future. Proponents of the SALT II breakout disputed this view, however, and argued that the Soviets will deploy fewer additional weapons in response to the U.S. move.

The bomber will be deployed without any corresponding "retirements or dismantlements" of Poseidons, an administration official said.

Spurgeon Keeny, president of the private Arms Control Association, said last night, "It is just outrageous that they have decided to violate the only limitations on strategic offensive weapons at a time when the prospects for any new agreements during Reagan's presidency have practically vanished."

But Sen. Dan Quayle (R-Ind.), a critic of SALT II, said that "this action is six years overdue, and given Soviet cheating . . . it is right on target."

In another arms control development yesterday, U.S. and Soviet officials said that no progress had been made in the latest round of superpower talks on nuclear weapons testing, which concluded on Tuesday.

Chief Soviet delegate Andronik Petrosyants said at a Geneva news conference that the Soviets had proposed the start-up of talks there on the "complete and final prohibition of nuclear explosions."

"Our constructive position drew no response from the United States side," he said. But U.S. officials said the two sides merely disagreed on the precise agenda for the talks and held out hope that an agreement could eventually be reached.