The Justice Department has warned Michael Orlando, a gangland informant who played a crucial role in the case against former labor secretary Raymond J. Donovan, that he may be dropped from the government's witness protection program because he has failed to cooperate with the Donovan defense.

Paul Coffey, deputy chief of the Justice Department's organized crime and racketeering section, made the warning in a letter last Friday to Orlando's lawyer, David Gould.

It came after Orlando's decision to invoke his Fifth Amendment privilege against self-incrimination when defense lawyers subpoenaed him to testify at pretrial hearings on the Donovan case in Bronx Supreme Court in New York.

Gould protested yesterday that banishing Orlando from the witness protection program would amount to signing his death warrant in light of many murder threats reportedly made against him. The lawyer said he knew of no obligation on Orlando's part to cooperate with the defense. Gould said Bronx prosecutors had no objection to Orlando's taking the Fifth.

In his letter, Coffey said: "Mr. Orlando was and is required to cooperate fully and truthfully with the parties in People v. Schiavone the Donovan case in order to maintain his status as a federally protected witness. Failure to provide such cooperation may result in termination of your client from the program."

Gould said he advised Orlando not to testify, largely because of signals that Justice Department and FBI officials, including Coffey, were bent on indicting Orlando again instead of "cleaning up the mess" now on their hands.

Now serving a federal prison term for truck hijacking, Orlando was the key informant for the Federal Bureau of Investigation in an ambitious 1979 organized-crime probe that produced extensive evidence of political corruption but no such prosecutions.

Bronx District Attorney Mario Merola subsequently obtained the FBI tapes compiled in the investigation and directed the inquiry that led to Donovan's fraud indictment under New York law. The hearings now under way were called to determine the tapes' validity.

Gould charged that "people in federal law enforcment have done everything they can to impede the Bronx investigation."

Coffey said only, "We don't make agreements that require witnesses to cooperate only with one side." He declined to say whether the government has a policy prohibiting protected witnesses from invoking the Fifth Amendment.