Former White House official Lewis "Scooter" Libby has been told he cannot delay starting his jail term while he appeals against his conviction.
Libby's supporters have called on President Bush to pardon him
Libby was found guilty of obstruction of justice and perjury over the inquiry into the unmasking of CIA agent Valerie Plame and sentenced to 30 months' jail.
The ruling means Libby could go to jail within weeks. His lawyers will seek to block the decision in an appeals court.
Libby was the former chief of staff to Vice-President Dick Cheney.
Nobody has ever been charged with the offence of leaking the identity of Ms Plame, whose husband criticised the Iraq war.
Ahead of the hearing, US District Judge Reggie B Walton had indicated he saw no reason to grant Libby's request for a delay in starting his jail term.
Giving his ruling on Thursday, he said: "Unless the Court of Appeals overturns my decision, he will have to report."
Libby's lawyers will now go to an appeals court to seek an emergency order delaying his prison sentence. They are also appealing against Libby's conviction.
It will be down to the Bureau of Prisons to decide when and where Libby should report to go to jail.
WHAT IS CIA LEAK CASE ABOUT?
Libby was found guilty of lying to the FBI and a grand jury over revelations about CIA agent Valerie Plame's identity
Critics said the White House leaked Ms Plame's identity to undermine her husband, ex-ambassador Joseph Wilson
He had publicly cast doubt on the Bush administration's case for war in Iraq
The alleged cover-up, rather than the leak itself, was the subject of the Libby trial
Libby's wife, Harriet Grant, wiped away tears as the decision was given but her husband remained stoic, according to reporters in the court. Libby left the building without commenting.
Before giving his ruling, Judge Walton said he had received "harassing, mean-spirited" letters and phone calls, targeting him and his family, since sentencing Libby to jail.
At the sentencing earlier this month, Judge Walton said the evidence overwhelmingly proved Libby's guilt.
He also fined Libby $250,000 (£125,000) and placed him on probation for two years following his release from prison.
There have been calls from Libby's supporters for President George W Bush to pardon him but the White House has so far been reluctant to get involved.
Responding to the ruling, White House spokeswoman Dana Perino said: "Scooter Libby still has the right to appeal, and therefore the president will continue not to intervene in the judicial process.
"The president feels terribly for Scooter, his wife and their young children, and all that they're going through."
Libby's lawyers have questioned whether Special Prosecutor Patrick Fitzgerald had the authority to bring the case against him in the first place.
They also argue that Libby was not able to present a full defence because classified information could not be discussed in court.
Libby, who has always maintained his innocence, argued that his long history of public service should be taken into account in sentencing him.
He is the highest-ranking White House official convicted in a government scandal since the Iran-Contra affair in the mid-1980s, when Ronald Reagan was president.
Ms Plame's identity as an undercover CIA agent was revealed in 2003 after her husband - a former ambassador - openly criticised the Bush administration's case for war with Iraq.
Libby was convicted of lying to FBI investigators and the grand jury about how and when he learned that Ms Plame was a CIA officer, and lying about disclosing classified information to reporters.