UBS is considering whether to reveal the names of up to 20,000 wealthy American clients as US authorities step up their investigation of alleged tax irregularities at the Swiss wealth manager, according to reports.

US investigators believe private bankers at UBS may have helped US clients hide as much as $20 billion in offshore accounts to avoid paying tax, the New York Times reports, citing people close to the inquiry.

The US believes the alleged scam may have cost up to $300 million in lost taxes, according to a Government official.

A spokesman for UBS declined to comment on the report and said the bank’s position on the US investigation had not changed.

In May, the bank revealed that the Department of Justice and the Securities and Exchange Commission were examining UBS's conduct in relation to Swiss-based advisers to US clients over a seven year period from 2000 to 2007.

"UBS is however treating these investigations with the utmost seriousness and has committed substantial resources. UBS intends to appropriately and responsibly address and correct any issues raised in the investigations," it said last month.

The Swiss bank has been under intense pressure since the tax inquiry began last month and has already had one of its most senior private bankers detained in the US.

Martin Liechti, head of the bank’s North and South American wealth management operations, was arrested as a material witness last month. He was released without charge the same day.

Bradley Birkenfield, 43, a former director in UBS’ wealth management division, has already been charged with helping a US client evade taxes.

Mr Birkenfield, a US citizen who worked for UBS in Switzerland, faces a maximum five year jail term, and is alleged to have helped US investor Igor Olenicoff hide around $200 million from US tax authorities.

He has indicated that he intends to plead guilty to a single charge of conspiring to defraud the United States.

Olienicoff, a real estate tycoon whose wealth is estimated at $1.6 billion by Forbes magazine, pleaded guilty to tax charges last year agreeing to pay $52 million in back charges, penalties and interest.

Another banker, Leichtenstein-based Mario Staggi, was charged at the same time as Mr Birkenfield in relation to work done for Olenicoff. Mr Staggi has been declared a fugitive by the US Government.

The Securities and Exchange Commission, the US financial regulator, is conducting a separate investigation into UBS’ work for US clients.

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