The Justice Department has told President Reagan that he must support Richard M. Nixon's claim of executive privilege in Nixon's attempt to block the National Archives from releasing more than 2 million pages of his presidential papers.

In what would be a significant expansion of the doctrine of executive privilege, the department's unpublished legal opinion says that an incumbent president is always bound to support such claims by a former president, except in rare cases where it would conflict with the incumbent's constitutional duties.

Congressional critics say the department opinion would enable Reagan and future presidents to keep their own papers under seal years after they have left office. The opinion says that the director of the National Archives, which Congress made an independent agency last year, has no choice but to comply with an executive-privilege claim by an incumbent president.

The House Government Operations subcommittee on government information, which plans hearings on the issue today, provided a copy of the opinion. Subcommittee Chairman Glenn English (D-Okla.) called it "highly suspicious" and said the Reagan administration is seeking "to establish a precedent here that would protect not only President Nixon but would also protect President Reagan in the future."

"Under this precedent, if it's allowed to stand, whoever is president would be in a position of supporting President Reagan's claim that his papers should be covered by executive privilege," English said. "We would establish a precedent that every president has to respect a claim by a former president indefinitely."

The 30-page opinion, signed by Assistant Attorney General Charles J. Cooper, says that "a former president's privilege would be of little value if it were dependent upon the ratification of his successors . . . . As a general matter, an incumbent president should respect a former president's claim of executive privilege without judging the validity of the claim."

The latest twist in this long-running dispute comes 12 years after Congress, in the wake of the Watergate scandal, passed a law to make public 14 million pages of documents from the Nixon White House and 4,000 hours of Nixon tape recordings. The executive branch has tried five times to adopt regulations to process this material, which is stored in a warehouse in Alexandria, but each attempt has been stymied by litigation or internal disagreements.

The archives had hoped to release by June 2.5 million pages of Nixon documents, including 1.5 million pages of politically sensitive "special files" kept by such former Nixon aides as H.R. Haldeman, John D. Ehrlichman and John Dean. But the dispute over the latest proposed regulations may set back that timetable.

The latest proposed rules give the nation's chief archivist the power to decide claims by Nixon and his former aides that some material should be withheld from the public on privacy or other grounds. But Cooper's Office of Legal Counsel says the archivist is a presidential appointee and must comply with the president's claim of privilege, even when it is invoked in support of a former president.

"If it were otherwise, there really wouldn't be anything to a former president being able to claim executive privilege," Cooper said in an interview. "The archivist must respect the claim of privilege by a former president." But English said the opinion "raises serious questions about the independence of the National Archives."

Cooper said the opinion was requested by the Office of Management and Budget, which approved the rules in February, and is binding on the executive branch. He said that, in any event, the archivist's decisions are subject to court review and are likely to be challenged by former Nixon aides, who have already succeeded in overturning one set of regulations.

All presidents before Nixon took their papers with them when they left office, but under a 1978 law Reagan's documents and those of his successors will become public property. That law requires the archivist to decide which papers to withhold and which to make public 12 years after a president leaves office.

Cooper dismissed suggestions that the opinion was drafted with Reagan's papers in mind. "I think executive privilege is for the benefit of all presidents, and surely will benefit Reagan as well as any other," he said.

White House officials have said that any former Nixon aides in the Reagan administration would disqualify themselves from the issue. But a memo obtained by English's subcommittee shows that the proposed rules were sent for review to White House deputy counsel Richard A. Hauser, who worked in the Nixon White House.