The May 2006 letter, now in the hands of the US Department of Justice, was written by Peter Kurer, UBS chairman and then the bank’s general counsel, and copied to Marcel Rohner, then head of private banking and now group chief executive, as well as Lawrence Weinbach, a UBS director who sits on the board’s audit committee.

The letter was written to Bradley Birkenfeld, a former UBS banker at the centre of US inquiries into whether the bank helped its wealthy US clients evade taxes by transferring assets to European tax havens.

Mr Birkenfeld, who admitted this year to helping a billionaire US businessman evade millions in tax while at UBS, has been co-operating with the authorities.

It was not until May this year that the Justice Department and the Securities and Exchange Commission revealed they were investigating the tax issue as well as claims that UBS bankers had advised US clients on investing in securities without obtaining the US registration legally required to offer such advice.

In the letter, Mr Kurer acknowledged he had received information from a whistleblower, who had drawn attention to the problems the bank faced because of the inadequate securities registration. Mr Kurer writes that the information prompted an internal investigation in which UBS interviewed 12 of its private bankers who had been dealing with wealthy US clients and he would be recommending procedural changes to management.

“I thank you for drawing my attention to this compliance issue,” Mr Kurer wrote. He added that in keeping with bank policy, the whistleblower “must not fear any retaliation”.

UBS officials stress the whistleblower referred only to possible breaches of securities law, and not the tax evasion issues being investigated. UBS also said Mr Kurer acted swiftly to tighten procedures after investigating the whistleblower’s allegations, and introduced measures to ensure the procedures were properly implemented and observed.

However, former members of the bank’s US offshore team say that in spite of these elaborate formal compliance procedures, bankers still faced enormous pressure to win business.

For example, the procedures that Mr Kurer introduced meant bankers faced a painstaking approval process before going to see customers in the US to ensure no laws would be broken.

But bankers were also told they should go to the US at least four times a year to meet existing clients, often in informal situations such as bank-sponsored arts or sporting events, that would double as business meetings and provided the opportunity to meet new customers.

UBS last November decided to close its offshore banking business for US clients after growing concerns about risks to the bank’s reputation in an ever-tougher regulatory climate.