Three journalists asked the Supreme Court on Tuesday to block an order requiring them to reveal confidential sources in their reporting on a criminal probe of a nuclear scientist.

If justices accept it, the case sets the stage for a landmark free-speech ruling at a time reporters are under increasing pressure to break source confidentiality pledges — or risk jail time and huge fines.

"The issues presented here go to the heart of the press's function in a democracy," justices were told in an appeal being filed Tuesday by lawyers for H. Josef Hebert of The Associated Press, James Risen of The New York Times and Robert Drogin of the Los Angeles Times.

The three reporters have been held in contempt of court for refusing to divulge the identify of sources for stories about Wen Ho Lee, who in 1999 was suspected of spying while he worked at Los Alamos National Laboratory in New Mexico.

Lee was never charged with espionage. He was held in solitary confinement for nine months, then released in 2000 after pleading guilty to a single count of mishandling computer files. A judge apologized for Lee's treatment.

Lee wants the reporters' sources for his civil lawsuit against the government, which he contends improperly disclosed personal information about him in violation of a federal privacy law. Justices could use the case to clarify how courts should deal with subpoena requests and decide if journalists are protected by the First Amendment in keeping sources private.

Reporters have been pressed to testify in criminal cases as well. Last summer, several reporters were subpoenaed to testify before a grand jury investigating the leak of CIA officer's identity. New York Times reporter Judith Miller spent 85 days in jail for refusing to discuss her source.

"Resolution by this court of this important question is particularly necessary because reporters across the country are now experiencing an ever-growing wave of federal subpoenas demanding the disclosure of their confidential sources, particularly in civil proceedings," justices were told in the appeal by Washington lawyer Lee Levine.

Levine said that about 20 reporters have recently faced subpoenas to testify in two cases that involve the federal Privacy Act, six in the Lee case and another dozen in a lawsuit brought by Steven Hatfill, a former Army scientist who claims journalists unfairly linked him to the 2001 anthrax killings.

The New York Times has an appeal pending at the Supreme Court seeking to throw out a defamation lawsuit brought by Hatfill.

A separate appeal was expected to be filed on behalf of a fourth reporter also ordered to testify about confidential sources involving Lee — Pierre Thomas, formerly of CNN and now of ABC.

The case is Drogin v. Lee.