The Justice Department admiitted yesterday that in 1975 the government declassified a report that revealed "the essential secret of the H-bomb."

The department also admitted that a competent scientist who had read that report would get "no significant additional information or benefit" from an article and diagrams that the Progressive magazine has been prevented from publishing.

For 34 months, the document - "UCRL-5280" - was accessible to anyone at the library of the Los Alamos Scientific Laboratory in New Mexico, which has the largest single collection of public materials on hydrogen bombs. It was listed in the card catalogue and kept on the open shelves of the library.

The admissions were made in a stipulation signed by Thomas S. Martin, deputy assistant attorney general, and an attorney for the American Civil Liberties Union, which represents Progressive editors Erwin Knoll and Samuel H. Day Jr. The other signers were lawyers for the magazine itself and for Howard Morland, who wrote the article.

The stipulation was filed in Milwaukee with U.S. District Court Judge Robert W. Warren, who will hold a hearing Tuesday on motions by the ACLU to admit new evidence on government breaches of its own security on H-bombs and to lift Warren's preliminary injunction against publication of the Progressive article.

The injunction - the first in American history to impose a prior restraint on publication - was issued at the government's request March 26.

Since then, the government's case has suffered sharp blows. One of the most severe came last month, when an ACLU researcher went to the open shelves in Los Alamos and copied another declassified document, UCRL-4725, to which there had been unrestricted access for four years.

A former nuclear weapons designer, Theodore B. Taylor, testified May 23 that the declassification of UCRL-4725 created "the most serious breach of security" in the history of the nation's nuclear weapons development program.

The Senate subcommittee on nuclear proliferation, before which Taylor appeared, then disclosed that the old Atomic Energy Commission had declassified - erroneously - not only UCRL-4725, but also UCRL-5280, the report now acknowledged to reveal "in a thermonuclear context the three concepts which the government has described as the essential secret of the H-bomb."

Still another blow to the government's case came yesterday, when a subcommittee source disclosed that the government had declassified erroneously an unspecified number of additional reports containing "sensitive" H-Bomb information.

Each of these declassifications was "a serious breach" of security, the source told a reporter.

In addition to these developments and growing support of the Progressive from newspapers and press organizations, the Los Angeles Times reported Thursday that most Justice Department lawyers assigned to the case want to abandon it.

But a department spokesman said yesterday that the case won't be dropped. "The position that we originally took was the correct one, and we intend to maintain it," he said.

Energy Secretary James R. Schlesinger, a former AEC chairman, reportedly took a personal interest in launching the government's quest for the preliminary injunction.

One of the ACLU's key arguments in fighting the injunction is that the article contains nothing nearly so useful to a would-be bomb builder as the mistakenly declassfied documents made available at Los Alamos. There, library users - whetther American or foreign - could photocopy what they wished with no records being kept.

After the erroneous declassification of UCRL-5280 was disclosed by Sen. Sen. John Glenn (D-Ohio) at the sub-committee hearing May 23, ACLU lawyers - who have been given security clearances - asked the government to let them examine the report in secrecy. The Justice Department agreed, but then, after DOE objected, reversed itself.

Protesting that its ability to prepare its case for the Progressive was being impaired by this and other government actions, the ACLU won the Justice Department's agreement to the stipulation signed yesterday.

The stipulation will "facilitate the court's consideration of the pending motion to vacate the preliminary injunction," the department's Martin told Judge Warren.

In declassifying UCRL-4725, the intent of the AEC was to make public only a few pages on nuclear propulsion of spacecraft, and to keep secret the bulk of the report on nuclear weapons.

By contrast, UCRL-5280 was declassified in its entirety despite the warning signal conveyed by its title: "Weapon Development During June 1958."

The declassification directive from the AEC in Washington to its Technical Information Center in Oak Ridge, Tenn., was "absolutely unambiguous," a subcommittee source told a reporter after the May hearing. It raises the possibility of "negligence up and down the system," he said.