One day this summer, Michael Bohndorf, a 69-year-old Deutsche Bank shareholder, travelled from his home on the Spanish island of Ibiza for a meeting in Frankfurt with one of the bank's lawyers. There, he recalls, he received a "white gloves" welcome.

"I was treated like the prince of Peru," he says. "It was all 'How would you like your tea, Mr Bohndorf?' "

If Mr Bohndorf, a German lawyer, found the deference exaggerated, it was not surprising. As both he and the bank's own lawyer knew, the meeting was the consequence of much more unpleasant conduct by the bank: the use of private detectives to investigate him. In an operation that could have come from a spy novel, detectives allegedly arranged a holiday rental of the shareholder's house to try to unearth information. He also claims a woman was sent as a "honey trap" to glean material about him.