In an ironic twist, it was only because a Barclays employee blew the whistle that the bank’s board learnt of its chief executive’s ill-advised personal crusade to unmask another whistleblower.

The board discovered earlier this year that Jes Staley had breached the rules designed to protect whistleblowers’ anonymity after a member of staff raised concerns about the bank’s whistleblowing procedures. The tip-off set in train a series of events that now threatens to engulf Barclays, which already has a tarnished reputation, in yet another crisis.

The latest controversy began in June 2016, when two anonymous letters were sent from the US to some Barclays board members and another senior executive. It is understood they concerned Tim Main, a friend and former colleague of Staley’s from his 34 years at JP Morgan.

Main is a well-regarded banker and one of a number of ex-JP Morgan employees that the 60-year-old American Staley hired to help him turn Barclays around. The letters concerned issues of a personal nature about Main’s past, when he worked at the Wall Street firm, and questioned the appropriateness of his recruitment to Barclays, where he is now chairman of its investment bank’s financial institutions group.

They were “very simple, very crude”, and “very malicious”, a source close to the matter said, and contained no new information. Nevertheless, Barclays’ compliance team treated them as a whistleblowing matter and set about investigating the letters.