While Barclays chairman John McFarlane said Mr Staley, who has apologised for his actions, still had the "unanimous confidence" of the board, in the worst case the FCA could decide to ban the bank chief from financial services, which would mean he could no longer continue to lead Barclays.
The FCA and PRA can also impose fines on both Barclays and Mr Staley for the transgressions and force the bank to change its whistleblowing processes. In another blow to Barclays, it is understood that the Department of Financial Services in New York is also looking into the matter, which could lead to further action.
The Barclays board learnt of Mr Staley's pursuit of the whistleblower early this year and hired law firm Simmons & Simmons to undertake an independent review. The lawyers and board concluded that the chief executive "honestly, but mistakenly, believed that it was permissible" to identify the letters' author.
The financier who was the subject of the letters had "experienced personal difficulties in the past", Mr McFarlane said. It is understood the banker in question is Tim Main, who worked and befriended Mr Staley while they were colleagues at JP Morgan.
Mr Main was hired by Barclays last June, one of a host of former JP Morgan bankers to follow Mr Staley to the British lender to help him revive its fortunes since he took the helm late in 2015.
Mr McFarlane said Mr Staley wanted to track down the letter writer to "protect a colleague" who he believed had suffered an "unfair attack". In an email to staff today, the Barclays chief admitted he "got too personally involved" but insisted the whistleblower was seeking to "maliciously smear" their colleague.
The Barclays chairman said he was "personally very disappointed and apologetic that this situation has occurred", which serves to further tarnish Barclays' already battered reputation, John Mann, a Labour MP who sits on the Treasury Select Committee, urged Mr Staley to resign.