Bradley Birkenfeld, a key informant in the U.S. tax evasion case against Swiss bank UBS , was sentenced to three years and four months in prison on Friday by a federal court judge in Florida.

Birkenfeld, a U.S. citizen and former UBS banker, was sentenced for conspiring to defraud the United States by helping a billionaire U.S. real estate developer create sham corporations and entities to hide $200 million in assets from U.S. tax authorities.

The 40-month sentence was harsher than expected.

Prosecutors, citing Birkenfeld's extensive cooperation in a major U.S. government tax evasion probe against UBS, had asked this week that his sentence be reduced to 2-1/2 years imprisonment rather than the five years he could have faced.

Birkenfeld had been credited with providing insider information in a sweeping U.S. investigation of UBS over its private banking business, involving wealthy Americans who used their Swiss accounts to hide money overseas to evade taxes.

His sentencing, delayed four times since it was originally set for August last year, came two days after U.S. and Swiss authorities signed a pact in which Switzerland agreed to reveal the names of about 4,450 wealthy American clients of UBS to U.S. authorities.

Birkenfeld pleaded guilty in June 2008 to helping real estate mogul Igor Olenicoff conceal $200 million in assets from U.S. tax authorities.

(Reporting by Tom Brown; Editing by Pascal Fletcher and Tim Dobbyn)