Barclays chief executive Jes Staley has been fined £642,430 by regulators for breaching rules by trying to identify a whistleblower.
The Financial Conduct Authority and the Prudential Regulation Authority said he failed to “act with due skill, care and diligence” in his response to an anonymous letter received in June 2016.
The FCA and PRA began their probe into Mr Staley’s conduct a year ago.
Barclays said it would cut his bonus by £500,000.
Directors delayed deciding how much Mr Staley’s bonus would be docked until the investigation was complete and they knew the penalty imposed.
Mr Staley said: “I have consistently acknowledged that my personal involvement in this matter was inappropriate, and I have apologised for mistakes which I made.
“I accept the conclusions of the board, the FCA, and the PRA, following their respective investigations, and the sanctions which they have each applied.”
Mr Staley earned £2.35m in 2016 and received a bonus of £1.3m.
The fine amounts to about a fifth of his total compensation. It would have been more than £900,000, but he was given a 30% discount for settling at an early stage.