WASHINGTON — Special Prosecutor Patrick Fitzgerald urged a federal judge yesterday not to delay the two-year prison sentence of a former White House aide, I. Lewis “Scooter” Libby, in the CIA leak case.

Libby, the former chief of staff to Vice President Cheney, has argued that he has a good chance of winning an appeal and should be allowed to remain free until that challenge has run its course.

U.S. District Judge Reggie Walton, who sentenced Libby to prison for lying to authorities and obstructing the investigation into the 2003 leak of CIA operative Valerie Plame’s identity, has said he sees no reason to grant Libby’s request.

He did not set a date for Libby to report to prison, however, and scheduled a hearing on the issue for tomorrow.

A delay in Libby ‘s sentence would give President Bush more time to consider pardon requests from Libby’s supporters, who say the loyal aide was caught up in a political investigation and does not deserve prison time.

Mr. Fitzgerald, in court documents filed yesterday, said an appeals court is unlikely to overturn Libby’s conviction because the evidence against him was so overwhelming.

After a month-long trial, jurors found that Libby lied to investigators about how he learned that Ms. Plame, the wife of an outspoken war critic, worked for the CIA, and whom he told.