President Reagan is close to making an important policy decision under which the United States essentially would propose to continue observing the restraints of previous nuclear arms control agreements with the Soviet Union as long as Moscow did, according to administration officials.

The idea, officials say, is to provide interim arms restraints while new Strategic Arms Reduction Talks (START) get under way.

The decision, other sources say, would also end bickering within the administration over whether to continue abiding by certain restrictions in the SALT I and SALT II strategic arms agreements with Moscow. The bickering and seemingly conflicting public statements on the question have created confusion here and among U.S. allies as to the American position on the two SALT agreements.

The president is expected to say something publicly on the interim arms control issue before leaving on his trip to Western Europe next week. Before Reagan's statement, the Senate Foreign Relations Committee is expected to put forward a resolution in support of the president's START plan but also calling for retention of existing arms control restraints as talks proceed.

President Reagan has called for START talks to begin late next month, and Soviet leader Leonid I. Brezhnev reportedly has welcomed the idea, even while rejecting some initial U.S. proposals on how to reduce forces.

The effect of the expected presidential decision, assuming Soviet agreement, would be to keep intact, at least for the time being, the restrictions on missile forces that the two superpowers agreed to in the first U.S.-Soviet strategic arms agreement signed in 1972 and a second agreement signed in 1979.

The initial five-year SALT I agreement has expired, and SALT II was never ratified by the U.S. Senate.

But the combined effect of the agreements has been to limit the land-based and submarine-based missile launchers in each arsenal and the individual warheads allowed on each missile. The agreements also rule out such things as construction of new underground missile silos or deployment of mobile missiles, which could roam the countryside and escape detection by picture-taking satellites.

The Reagan administration has rejected proposals that the United States ratify SALT II as an interim step, because it says that agreement gives Moscow a big edge in very large land-based missiles. Ratification, in the administration's view, would reduce Moscow's incentive to negotiate big reductions in those forces.

Administration officials, however, including Secretary of State Alexander M. Haig, Jr., have said the United States is not undercutting the ceilings and other constraints in the SALT agreements. Officials have said this in the past, when the administration was still reviewing its arms control strategy.

Now, the idea is to make this official presidential policy. It is not clear, however, how long such interim restraints would run.

It would be to Washington's advantage to get such restraints adopted because it would keep Moscow from deploying new weapons while the United States could go ahead with development of new weapon systems.

Officials here say the first real test of the new policy will come soon. The Navy's newest Trident missile-firing submarine, the USS Michigan, is scheduled for sea trials this month, and under SALT I an older submarine should come out of service. Haig recently told the Senate committee this would take place, but it is known that this was the source of some heated debate with the Navy.

Also, the first B52 bombers are being loaded with new cruise missiles, and SALT restricts the number allowed on each plane.