UPDATE 4-US says building criminal cases against UBS clients

* More than 150 UBS client criminal cases already in works

* Probe targets "multibillion-dollar scheme to defraud US"

* Ex-banker sought to "peel back curtain" on bank secrecy

* Wider settlement details due to be announced Wednesday (Adds separate court filing by Birkenfeld)

By Tom Brown

MIAMI, Aug 18 (Reuters) - 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 (UBSN.VX)(UBS.N).

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. [ID:nLH57374]

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.

The U.S. case against UBS, which has been closely watched by the global offshore banking industry, has challenged Switzerland's tradition of bank secrecy and confidentiality.

In his own separate court filing, Birkenfeld said he was the first known banker operating in Switzerland who sought to "peel back the curtain" of its bank secrecy laws.

He also said his extensive cooperation with U.S. authorities, beginning as a whistle-blower in 2006, had reportedly led the United States to extend its inquiry into illicit activity by other foreign banks.

Based largely on his information, UBS could have been forced to give the U.S. Internal Revenue Service the names of 19,000 U.S. clients suspected of tax evasion, he said.

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, prosecutors said in Tuesday's court filing.