Three East German spies arrested here last year had access to hundreds of top-secret West German and NATO military documents and presumably passed them on to the Communist bloc, Defense Ministry officials acknowledged today.

Details of the operation, possibly the most damaging spy case here in many years, were reported today by the Frankfurter Allgemeine newspaper. The magnitude of the secrets betrayed in the plot, the paper said, sur-passed the celebrated 1974 case of Guenther Guillaume, an East Germany spy who had become a close aide of then Chancellor Willy Brandt.

Defense Ministry spokesman Kurt Fischer, who confirmed much of the Frankfurter Allgemeine's account, called the new case "exceptionally serious." The alert plans of the West German army and sensitive North Atlantic Treaty Organization analyses reportedly were among the documents compromised.

Meanwhile, West German readers are also being told that it was not only the presence of a spy in the chancellory that brought about the resignation of Brandt in 1974.

Brandt, according to a new book by the former federal security chief that is being sertalized here by the news magazine Der Spiegel, also feared being blackmailed by Guillaume and East Germany over details of his alleged extramrital affairs.

West Germany traditionally has been a lucrative and relatively easy target for thousands of Communist agents, mostly East Germans who can blend in easily and who speak the language. Millions of people have fled here from Communist countries since World War II, among them many intelligence agents unwittingly welcomed in the West as refugees.

It is estimated that there are close to a thousand top quality spies operating in West Germany and between 6,000 and 15,000 lower levelsagents or contacts.

The massive spy problem in recent years causea the United States to be reluctant to pass some highly sensitive materials to Bonn. The West German security apparatus is viewed as considerably improved lately, however.

Despite the apparent dimensions of the current scandal, it may not be the worst. In 1961, for example, the chief of West Germany's Soviet counter-intelligence branch was discovered to be a spy.

The three alleged spies in the current case - Renate Lutze, a former secretary in the Defense Ministry; her husband, Lothar-Erwin, who worked in the ministry's weapons section and a friend, Juergen Wiegel, who worked on the naval staff - were arrested in June 1976 in a round-up of some 15 suspected spies operating here.

At the time, informed allied intelligence sources said privately that the round-up was one of the most important in years and that concern was greatest about the small group linked to the Defense Ministry.

The round-up was also viewed then as a major success for Bonn's counter-intelligence force. The details reported today indicate, however, that the damage done was considerable.

The newspaper, citing an interim report of the federal prosecutor's office, said that files apparently passed on to the East Germans included the West Germany army's plans for dealings with a military crisis, ammunition distribution plans, civilian avacuation plans and future development plans for the West Germany army and its designs for tanks beyond the newest Leopard models.

The three reportedly had access to more than "a thousand documents," about a fifth of them dealing with NATO and reportedly including Western analysis of Warsaw Pact forces, critiques of recent NATO exercises, and NATO emergency fuel distribution plans.

Defense Ministry officials acknowledged that the material evidently passed on to Communist bloc countries probably did deal with a number of these subjects.

The key figure reportedly was Mrs. Lutze, who apparently was able to copy files in the office of her former boss, the head of the social welfare department in the Defense Ministry.

Why so many highly classified files should be in that department was partially explained by officials as a result of the department head's position on a committee that dealt with other military matters. Nevertheless, there was some suspicion that the files had been ordered in an unauthorized fashion by someone else.

Guillaume, who arrest led to Brandt's resignation, was an East German military officer who was planted in the West in 1956 and eventually worked his way into Brandt's inner circle. He was arrested, with his wife, in April, 1974. Two weeks later, Brandt resigned over the furor, admitting he was negligent in allowing Guillaume to occupy such a position.

At the time, there were numerous rumors, denied by Brandt, that the resignation was also tinged with fears that Guillaume knew of private Brandt escapades that could lead to blackmail.

The new book by Guenther Nollau, who resigned in 1975 as head of the Federal Office for Protection of the Constitution, is the first account by a public figure that appears to lend credence to these rumors.

The book gives no specific details, but Nollau says he first heard of these alleged affairs from Federal Criminal Office chief Horst Herold about 10 days after Guillaume's arrest.

Herold's office had learned them from Brandt's bodyguard, Nollau reports.

The former official then reports that he told Herald that "if Guillaume dishes up these piquant detals at his trial, the government and the Federal Republic will be made fools of.

"If he doesn't say anything, the East German government, whom Guillaume has naturally told all about it, too, has the means to humiliate any Brandt Cabinet and the Social Democratic Party."

This information was passed on to Herbert Wehner, the powerful floor leader in Parliament of Brandt's Social Democrats. After talks with Wehner and other top figures, Brandt resigned three days later, Nollau wrote.

In December 1975, Guillaume was sentenced to 13 years in jail. At his trial in Duesseldorf, in which Brandt was a witness, there was no mention of any transgressions on Brandt's part.