Senior executives at UBS, the Swiss bank being investigated by US authorities, knew some of their bankers had acted in a way that meant they risked breaching American securities laws at least a year before the US inquiries began, a letter seen by the Financial Times shows.

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.