President Reagan deferred a decision yesterday on whether the United States should continue abiding by provisions of the unratified SALT II arms control agreement, but a consensus appeared to be developing among his advisers on what one called "a gray-area approach" that would compromise conflicting positions of the Defense and State departments.

Administration sources said that, at a 75-minute National Security Council meeting yesterday, both departments and the Joint Chiefs of Staff recommended against dismantling a nuclear-armed Poseidon submarine to avoid breaking SALT II limitations when the new Trident submarine Alaska enters service later this year.

Secretary of State George P. Shultz contended, however, that the United States could comply with the spirit of the treaty by dry-docking the older Poseidon submarine for at least several months.

Arrangements for full compliance with SALT agreements between the United States and the Soviet Union would require that the Poseidon's firing tubes be dismantled within six months.

Under the proposal presented to Reagan, the United States would open the submarine's hatches and remove its nuclear missiles, partial steps that would leave the United States, according to some officials, only technically in violation of the treaty.

Shultz contended that this solution would be more acceptable to U.S. allies and avoid giving the Soviets "a propaganda advantage" that would be afforded through outright violation of the treaty, sources said.

But Defense Secretary Caspar W. Weinberger and Attorney General Edwin Meese III, also an NSC member, argued that Reagan should openly announce that he is no longer abiding by SALT II because of repeated Soviet violations of it. The administration contends that the Soviets have violated the treaty on four specific counts.

Neither Reagan nor national security affairs adviser Robert C. McFarlane gave their views in the NSC meeting. But a spokesman said later that the president will make no decision until Shultz meets Thursday with foreign ministers of the North Atlantic Treaty Organization in Portugal.

The ministers of the 16-nation alliance are expected to favor continued adherence to the treaty.

Reagan received a taste of likely European opinion yesterday when David Owen, leader of the British Social Democratic Party, visited the White House and strongly urged U.S. compliance with the treaty limits. Owen said he told the president that it is important to respect the SALT II limits "as long as there is any life in Geneva," where the United States and the Soviet Union are engaged in complex arms-control negotiations.

"It would be an immense help if the signal was that you are prepared to stay with the SALT II limits," Owen told Reagan. "I hope you will put the submarine in dry dock at the very least if you are not prepared to dismantle it."

Reagan, who said last month that there is "considerable evidence that voluntary compliance has been one-sided," is under pressure from conservatives to scrap the treaty openly. But one official said yesterday that the conservatives would be satisfied if Reagan decommissioned a submarine without dismantling it.

The issue has been forced upon the administration by the forthcoming sea trials of the Alaska, whose 24 intercontinental ballistic missiles will put the United States over the SALT II ceiling of 1,200 multiple-warhead ICBMs. The trials, originally scheduled for August, may be postponed until October, officials said.

Administration officials said Reagan would announce his decision in a report to Congress next Monday after conferring with McFarlane. One official said Reagan wants to "preempt" action by the Senate, where several pending amendments to the defense authorization bill deal with continued SALT II compliance.

Senate Majority Leader Robert J. Dole (R-Kan.) said the amendments would have to be postponed until Reagan's report because some senators would refuse to address the subject until then. Senate Minority Leader Robert C. Byrd (D-W.Va.) said there is a "great deal of Democratic support" for continued compliance.

Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) also voiced cautious support for compliance, citing a "general value to keeping with the SALT II construct" but adding that Reagan should also demonstrate that "we are not blind to the Soviet violations."

For these reasons, Lugar said, he would be "not at all unhappy if things remained a little vague for a while."

The Senate uncertainty appeared to mirror Reagan's conflicts on the issue.

"I don't think he likes any of the options," one administration official said. "He'd like to honor our agreements, but he doesn't want the Soviet record of violations to stand there without something being done. None of the options are really good."

A senior State Department official said Shultz would be listening to allies' views at the NATO foreign ministers meeting and is "not going to explain a decision that has already been taken."