WASHINGTON, Dec. 21 (Xinhuanet) -- A U.S. federal judge has resigned from the court that oversees government surveillance in intelligence cases in protest at President George W. Bush's secret authorization of a domestic spying program, The Washington Post reported Wednesday.

    U.S. District Judge James Robertson, one of 11 members of the secret Foreign Intelligence Surveillance Court, sent a letter to Chief Justice John G. Roberts late on Monday notifying him of his resignation without providing an explanation, the report said.

    Robertson privately expressed deep concern that the warrantless surveillance program authorized by the president in 2001 was legally questionable and may have tainted the court's work, two associates familiar with the judge's decision were quoted as saying.

    Robertson was appointed to the federal bench in Washington by President Bill Clinton in 1994 and was later selected by the then-Chief Justice William H. Rehnquist to serve on the Foreign Intelligence Surveillance Act (FISA) court.

    Word of Robertson's resignation came as two Senate Republicans joined the calls of a number of Democrats for congressional investigations into the National Security Agency's warrantless interception of telephone calls and e-mails to overseas locations by US citizens suspected of links to terrorist groups.

    The senators questioned the legality of the operation and the extent to which the White House kept Congress informed, the report said. Senator Arlen Specter, chairman of the Senate Judiciary Committee, has promised hearings next year.

    The revelation of the program last week by the New York Times also spurred considerable debate among federal judges, including some who serve on the secret FISA court. For more than a quarter-century, the FISA court had been seen as the only body that could legally authorize secret surveillance of espionage and terrorism suspects. Enditem