A federal judge has ruled that the former head of a federal whistle-blower protection office could face at least one month in prison for withholding information from congressional investigators, a decision that could derail a plea deal with prosecutors.

Scott J. Bloch pleaded guilty in April to criminal contempt of Congress for withholding that he ordered private technicians to "scrub" computer files at the Office of Special Counsel in December 2006.

His sentencing was set for last July, but it was postponed after watchdog groups criticized an arrangement in which Bloch admitted guilt to a single misdemeanor charge that carries a sentence of up to six months in prison, and prosecutors did not oppose his request for probation.

Groups that advocate ethics in government said that probation would understate the effect of Bloch's abuses during George W. Bush's administration.

Bloch gained notoriety for ordering the office to erase all references to workplace discrimination based on sexual orientation, saying his office lacked the authority to protect gay and lesbian employees. He was removed from the office after a meeting with White House officials in October 2008.

Separately, the FBI launched an investigation into whether Bloch was trying to obstruct a Hatch Act inquiry into whether he mixed political and official activities, and the House Committee on Oversight and Government Reform interviewed him in March 2008.

Whistle-blower groups said without a strong Office of Special Counsel, workers in federal bureaucracies were left unprotected. The office has been run by a career appointee, and President Obama in December nominated Carolyn J. Lerner, a Washington civil rights and employment lawyer.

Late Wednesday, U.S. Magistrate Judge Deborah A. Robinson issued a ruling finding that "Congress expressly provided for a mandatory minimum sentence of one month."

She rejected arguments from both the government and Bloch that courts had discretion to impose a lesser sentence, as baseball player Miguel Tejada received last year for a similar offense of misleading Congress about using performance-enhancing drugs.

Bloch's lawyer, William Sullivan, said in a statement, "We are certainly disappointed in the ruling, but are gratified that the government supported our position that the statute does not require a mandatory sentence, and we will continue to pursue a just resolution of this matter."

Bloch has no criminal history and could face a sanction on his ability to practice law.