"Please rest assured that a detailed inquiry has been launched by the Board of Directors into these events and we are confident that the truth will emerge," the two leaders wrote, according to an internal memo seen by Reuters.

Zurich prosecutors have opened a criminal investigation into the incident last week in which private detectives allegedly shadowed Khan, newly named as UBS's wealth management co-head, and his wife.

(Reporting by Oliver Hirt and John Miller; Editing by Michael Shields)