MIAMI, US - An ex-employee of Swiss banking giant UBS on Friday must begin serving more than three years in prison, a federal court judge ruled Monday, denying the man's request to delay the prison sentence and shorten its duration.

United States District Judge William Zloch denied a petition by defendant Bradley Birkenfeld, who had argued that his whistleblower role in exposing massive fraud by UBS made him deserving of softer treatment than the three-years, four-months sentence he received for inciting UBS clients to commit tax fraud.

Birkenfeld blew the whistle on some 19,000 UBS clients who were said to have invested some 20 billion Swiss francs (S$27 billion) in secret Swiss accounts, information which could allow the US government to recoup several billion dollars in lost taxes.

But it did not earn him a reprieve in the prison sentence handed down for his role in aiding a California property developer hide some 200 million dollars worth of investments in Switzerland and Lichtenstein.

"I gave them the biggest tax fraud case in the world. I exposed 19,000 international criminals. And I'm going to jail for that?? he told CBS television in a broadcast that aired Sunday.

Birkenfeld was sentenced in August, just two days after UBS and the US and Swiss governments reached a landmark out-of-court settlement in which the company agreed to hand over the names of some 4,500 suspected tax evaders.

Birkenfeld's attorney Stephen Kohn told CBS that his client eventually could be entitled to up to 30 per cent of any taxes recovered by the US tax authorities.