A key legal decision puts the National Archives one step closer to letting the public see presidential papers that Richard M. Nixon wants kept secret.

The decision Tuesday by the U.S. Court of Appeals in Washington knocks down a Reagan administration attempt to force the Archives to honor automatically all Nixon claims of executive privilege.It restores a regulation that says an Archives review board will study disputes over executive privilege claimed by Nixon and that the former president must go to court if he wants to overturn a board decision.

Although 31/2 million pages of Nixon documents have been opened to researchers, Nixon and his lawyers have been able to withhold 150,000 documents by claiming they involved executive privilege, private political associations, personal property or personal privilege.

A representative of Nixon's lawyers spent three years in the Archives warehouse in Alexandria, Va., identifying documents to be excluded from public view. The requests for exemption from disclosure totaled 3,100 pages and the protected papers represent about 10 percent of the 1.5 million pages in the so-called Nixon Special Files, which contained the most sensitive documents.

The Justice Department did not decide immediately whether to carry the case to the Supreme Court. "We've got 90 days to make any decision," said Michael Carvin, a Justice Department lawyer who helped prepare the appeal of a lower court decision.