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. The number of criminal probes is widely expected to mushroom soon.

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 to uncovering a multibillion-dollar scheme to defraud the United States involving UBS bankers and their U.S. customers.

They cited what they called his substantial assistance in the investigation and prosecution of others involved in a case that has challenged Switzerland's tradition of bank secrecy and confidentiality.

In February, as part of a deferred prosecution agreement, UBS agreed to pay $780 million to settle charges that it had helped American clients evade taxes on about $20 billion concealed in offshore accounts.

Information obtained from UBS, as part of that agreement, led directly to the criminal investigations now open against more than 150 Americans across the United States, Tuesday's court filing said.

Four American clients of UBS, three in Florida and one in California, are already being prosecuted on criminal charges stemming from the February agreement.

Citing the broader settlement of the UBS case, 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.

5,000 NAMES

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.

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.

Birkenfeld, a U.S. citizen, was employed at UBS from 2001 through 2006 and served as a director of its private banking division.

He had numerous U.S. clients and famously acknowledged, in a statement of facts issued when he entered his guilty plea in June 2008, that he once smuggled a customer's diamonds into the United States in a toothpaste tube to avoid detection by the authorities.

He also said he and other UBS bankers helped wealthy clients set up sham entities to hold their assets in offshore financial centers, such as Switzerland, Panama, the British Virgin Islands, Hong Kong and Liechtenstein.

Birkenfeld's lawyer could not be reached for immediate comment on the request for a reduced prison term for his client. (Reporting by Tom Brown; Editing by Pascal Fletcher, Gary Hill)