Rich Americans are turning themselves in to the Internal Revenue Service (IRS) in droves as the US tax authority steps up its hunt for tax evaders.

Lawyers reported getting hundreds of calls from clients with offshore bank accounts that they have not declared to the IRS, seeking advice on whether to take advantage of an amnesty on tax cheats.

The IRS has given customers of UBS, the Swiss bank, until September 23 to own up to secret foreign bank accounts in return for a financial penalty and the promise of no jail time.

The tax authority underlined how aggressively it intends to pursue non-payers and their agents when the Department of Justice (DoJ) indicted Hansruedi Schumacher, a Swiss banker, and Matthias Rickenbach, a Swiss lawyer, for helping Americans hide their assets from the IRS.

The charges were based on confessions obtained from UBS clients suspected of cheating on tax, according to John DiCicco, acting assistant attorney general of the DoJ’s tax division.

He said: "We encourage foreign banks to come forward and disclose their conduct immediately, before we learn about their criminal conduct from US taxpayers".

Asher Rubinstein, an attorney at Rubinstein & Rubinstein in New York, said that he was advising clients, ranging from those with straightforward cash accounts to others with gold bullion, stock and other securities stashed offshore, to take advantage of the amnesty.

"Many more people are coming out to the woodwork now," he said. "More people are realising that there won’t be secrecy any more."

Doug Shulman, Commissioner of the IRS, declined to say how many people had identified themselves to the taxman under the amnesty but described the response as "unprecedented".

The amnesty opened in February when the DoJ, on behalf of the IRS, sued UBS to give up the names of thousands of its offshore clients that the authority suspected of evading tax.

Yesterday the IRS said that UBS had agreed to supply the names of 4,450 clients to the Swiss Government, which would decide how many to hand over to the US.

UBS will write to the clients whose names have been turned over to the Swiss Government, giving them a chance to take advantage of the amnesty before their name lands with the IRS.

"Once the Swiss Government sends us the name, all bets are off," Mr Shulman warned.

He said that the IRS would pursue other banks and financial advisers once it got information from the 4,450 people on how they hid their assets from the taxman.

The charges laid against Messrs Schumacher and Rickenbach demonstrate that the IRS does not intend to limit its investigations to foreign banks with interests in the US.

In the charges against the two men, the DoJ claims that they convinced clients to move their cash from UBS to Neue Zuercher Bank, a smaller Swiss private bank with no business in America.

The US Government’s legal action against UBS was a huge blow to Switzerland’s reputation for watertight bank secrecy.

But the settlement enabled the Swiss Government to yesterday offload its 8.5 per cent stake in the bank, obtained in return for a SwFr6 billion bailout last October, at a SwFr1.2 billion profit.

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