The United States is building criminal cases against more than 150 American clients of Swiss bank UBS as part of a crackdown on tax evasion now made easier by a deal over access to secret account information.

U.S. prosecutors gave their first official confirmation of the initial number of criminal investigations in a filing on Tuesday with a federal court in Fort Lauderdale, Florida.

In the same court document, the prosecutors requested a sharply reduced prison sentence for ex-UBS banker Bradley Birkenfeld, a key informant in the ongoing U.S. prosecutions of wealthy American clients of UBS.

The request was made a day before U.S. and Swiss authorities were due to announce details of a negotiated settlement of their legal dispute over access to further names of UBS clients suspected of cheating U.S. tax collectors.

In the court filing, prosecutors said evidence provided by Birkenfeld had been critical in obtaining information from UBS that directly led to the investigations into the more than 150 Americans who are believed to have concealed income and assets at UBS from U.S. tax authorities.

Citing the settlement, due to be finalized on Wednesday, the filing said: It is expected that ... UBS will produce the identities and account information of additional UBS customers who are believed to have violated United States law.

A U.S. legal source told Reuters on Monday that UBS was expected to give U.S. authorities the names of about 5,000 more Americans suspected of using the Swiss bank to evade taxes.

In the court filing, prosecutors said Birkenfeld, a U.S. citizen who will be sentenced on Friday, had faced up to five years imprisonment after pleading guilty in June 2008 to helping a U.S. billionaire hide $200 million in assets from U.S. tax authorities.

The prosecutors asked that Birkenfeld's sentence be cut to 2-1/2 years imprisonment, citing his cooperation in efforts to uncover what they called a multibillion-dollar scheme to defraud the United States.

UBS first became the target of U.S. probes in 2007 when Birkenfeld first began cooperating with U.S. authorities and helped them start building criminal and civil complaints against the bank and its customers.

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