Consumer activist Ralph Nader yesterday accused the Reagan administration of concealing a report that outlines a new assault on black-lung benefits, calling it a "secret plan" to resume a battle that the administration launched three years ago and abandoned after more than 5,000 coal miners marched on the White House.

"This report, which the Reagan administration refuses to release to the public, constitutes an unconscionable attack on crippled miners and widows," said Nader, who released copies at a news conference. "This is the group that Mr. Reagan has selected to balance the budget on."

The Labor Department quickly denied that it had any plans to cut black-lung benefits. "Nader's accusations smack of pure fiction and crass political motives," Undersecretary Ford B. Ford said in a statement that attempted to cast the administration as the savior of the black-lung program.

The document, written under contract for the Labor Department by a University of Utah medical professor and a lawyer who has represented coal companies in black-lung claims cases, suggests that the term "black lung" be discarded and that eligibility requirements for benefits be tightened significantly.

Although the report states that it does "not necessarily represent the official position" of the department, many of its recommendations mirror the administration's 1981 proposals to pare the $1.8 billion-a-year black-lung program.

For example, the report suggests adopting a more restrictive definition of pneumoconiosis, the medical term for an array of lung disorders informally known as black lung, and requiring X-ray evidence of the disease.

Congress has repeatedly rejected proposals to require X-ray proof of black lung, because not all forms of pneumoconiosis show up on chest X-rays. It also has resisted attempts to narrow the definition of the difficult-to-diagnose disease.

The report does not estimate how many of the 342,000 recipients of black-lung benefits would be affected by the changes. But in 1981 the administration estimated that as many as 88 percent of miners who claim the benefits "were either not disabled or else could not be proved to have black-lung disease."

"This is an attempt to start all over again and raise issues that have already been decided by Congress in the favor of miners," said Davitt McAteer, director of the Occupational Safety and Health Law Center.

Congress asked for the report three years ago, and it was to have been delivered by mid-1983. Congress still doesn't have the report, although it is marked "final" and dated May 13, 1983.

Labor officials attributed the delay in the delivery of the report to the time involved in coordinating it with the Department of Health and Human Services and other interested parties.

In the last three years, the Labor Department has tightened some eligibility requirements for black-lung benefits, eliminating several "presumptions" that were intended to help survivors of suspected victims who died before the disease could be diagnosed.

But Labor officials yesterday denied that additional changes were in the works.

"To charge that the administration which saved the black-lung program from financial catastrophe three years ago now has some secret desire to cut that program is as irresponsible as falsely yelling 'Fire' in a movie theater," said Ford, who contended that the Reagan administration "put together the coalition which saved and reformed the black-lung program in 1981."

But the administration did propose cuts in the program as part of its initial budget-cutting sweep. By mid-1981, in the face of stiff congressional opposition and the threat of a national walkout by the United Mine Workers, the administration backed off the proposal in favor of increasing contributions to the program from coal companies.