The Air Force has told veteran whistle-blower A. Ernest Fitzgerald that he will lose his security clearance unless he signs a secrecy agreement within 30 days, and that his work with a House oversight committee will be severely restricted.

Fitzgerald has refused to sign the agreement and could lose his job, which requires access to top-secret documents, if his security clearance is revoked. He received the warnings in letters from the Air Force shortly before 5 p.m. Thursday.

Fitzgerald described the secrecy agreement yesterday as a "gag order" that prevents employes from releasing "classified and classifiable" information.

Fitzgerald said the service has also terminated a 3-year-old agreement that allowed him to periodically assist in defense-fraud investigations by Rep. John D. Dingell (D-Mich.), chairman of the Energy and Commerce Committee and its oversight subcommittee.

"They clearly decided to shut me down and to shut Dingell down by putting the gag order on me," Fitzgerald said in an interview. "This could go to releasing information to congressmen and senators."

Dingell, in a recent letter, quoted Steven Garfinkle, the federal official in charge of the secrecy program, as saying that the White House authorized it to prevent "leaks."

Dingell also quoted Garfinkle as saying, at a meeting with Fitzgerald and a Dingell aide, that the new form was designed to "simplify going after someone."

The dispute is the latest chapter in a long-running battle between Fitzgerald and his Air Force superiors. Fitzgerald gained national attention in 1969 when he was fired by the Nixon administration for disclosing cost overruns on the C5A cargo plane. After a long legal battle, a federal court ordered Fitzgerald reinstated as a "management systems deputy" in 1982.

Lt. Gen. Claudius E. Watts III, the Air Force comptroller, told Fitzgerald in one letter that failure to sign the secrecy agreement "will result in suspension of your access to classified information and continuation of procedures leading to revocation of your security clearance." Officials say that 1.5 million Pentagon employes have signed the agreement, but Fitzgerald has refused since December to sign.

Sen. Charles E. Grassley (R-Iowa) said in a recent letter to Air Force Secretary Edward C. Aldridge Jr. that the secrecy forms could have a "chilling effect" on "legitimate disclosures of nonclassified information to Congress by government employes."

The second letter from Watts revokes a 1985 agreement between Dingell and former Air Force secretary Verne Orr that has allowed Fitzgerald to assist the congressman on investigations of the MX missile and other probes. "I will not include congressional assistance . . . in your work plan," Watts told Fitzgerald.

The Associated Press quoted Col. Peter Sloan, the Air Force spokesman, as saying that the change will not prevent Fitzgerald from working with Dingell's committee, but that "there is a difference in regard to the type of access he will receive. Congressional staffers get the same access under different procedures. It might take a little bit longer.

"When he goes into the field, he can no longer identify himself as a senior Air Force official, but will have to identify himself as a congressional staffer" when he is working with Dingell's panel, Sloan said.

Fitzgerald, who charged that a recent reorganization of his office left him with little to do, said the Air Force appears determined to silence him.