The programme allows US citizens to avoid criminal prosecution if they acknowledge evasion and agree to pay taxes and penalties.

The clients' actions are a boon for the IRS, which lacks the staff to go after about 20,000 US citizens who US authorities say worked with UBS private bankers to avoid taxes.

The moves come as UBS, US authorities and the Swiss government have accelerated talks aimed at releasing data on the offshore accounts of US taxpayers, people familiar with the matter said.

Meanwhile, the IRS is considering a national settlement that would speed up the process for UBS clients to come forward en masse. Likely to be based on a 2003 landmark offshore credit-card and tax-fraud deal, though with stiffer penalties, the settlement could obviate the need to gain the cooperation of the Swiss government, which has been reluctant to turn over private bank client information.

The developments amount to a coup for US authorities, who have spent the past two decades trying to crack offshore banking markets with only incremental progress.

Any progress by US authorities represents a blow for Switzerland, which depends heavily on its reputation for secrecy to support a private banking industry that is the lifeblood of the Swiss economy.

UBS is counting on its private banking unit, which provides services to wealthy individuals, to help dig it out of more than $46 billion in write-downs on soured mortgage investments.

A spokesman for UBS said the bank "is treating these still ongoing investigations with the utmost seriousness and will address and correct any issues raised in the investigations, including by taking appropriate disciplinary action."

The national settlement could be announced before the end of the year, though the transition of president-elect Barack Obama's administration could slow it down, according to people familiar with the situation. An IRS spokesman denied that any new program was coming soon.