The U.S. Justice Department is considering whether to ask a judge to reduce the jail sentence of Bradley Birkenfeld, the key informant in the U.S. tax fraud case against Swiss bank UBS AG, the New York Times said, citing a person briefed on the matter.

Anything he can provide to us that he hasn't in the past, in particular with regard to foreign governments, would factor into our decision on whether to issue a request for leniency, an unnamed senior government official briefed on the matter told the paper.

A Justice Department spokeswoman did not immediately respond to a Reuters email seeking comment that was sent outside of regular U.S. business hours.

Birkenfeld, who is due to start serving a federal prison term of three years and four months on Friday, has been hailed by whistleblower advocates and U.S. prosecutors alike as pivotal to the case against UBS, his former employer.

In a December 26 filing with the U.S. District Court in Florida where he was sentenced, Birkenfeld's attorneys asked that his prison time be postponed and that a hearing be scheduled for the judge to reconsider the sentence.

Birkenfeld pleaded guilty to a single fraud conspiracy count in June 2008, when he acknowledged helping his largest U.S. client hide assets from the Internal Revenue Service.

The Swiss bank was targeted in a wide probe by U.S. authorities for helping U.S. tax cheats to hide assets in UBS accounts.

(Reporting by Ajay Kamalakaran in Bangalore; Editing by Lincoln Feast)