UBS warned US customers by registered mail that their account details may be given to US tax authorities, a method that could itself breach secrecy laws, according to the Swiss weekly paper Sonntag.

The article said the use of registered mail and envelopes showing the sender was UBS could help US law enforcement and tax authorities discover those UBS clients suspected of tax evasion. This could happen long before their details are passed to the US under an agreement between the US and Swiss governments announced several weeks ago, the publication said.

In August, Switzerland and the US reached a settlement under which 4,450 UBS client details will be handed to the US authorities. Under the terms of the deal, the first names may not be transferred until early next year.

As reported last week, about 7,500 US citizens – including some UBS clients – have disclosed data about money held in offshore accounts. It is unclear as to how many more people have yet to disclose such information. A US voluntary disclosure programme ended on 15 October.

A translation of the Sonntag article by Reuters quoted lawyer Andreas Rued, who is representing some US clients of the bank, as saying the use of registered mail and envelopes showing the name of the bank could constitute a contravention of Switzerland's banking secrecy laws. He said he was considering whether to seek a criminal investigation against UBS.