Barclays has said it will fight the record fine imposed on it by a US energy regulator over the alleged manipulation of power markets in California and other states.
The British bank is facing a $453 million fine (345 million euros).
Hefty penalties have also been levied against four of its power traders who were accused of manipulating an index relating to electricity prices.
The Federal Energy Regulatory Commission (FERC) will likely now move the case to a federal court.
Barclays said it was “disappointed” by the Commission’s action and added: “We believe the penalty assessed by the FERC is without basis, and we strongly disagree with the allegations made.”
Spokesman Marc Hazelton said: “We have cooperated fully with the FERC investigation, which relates to trading activity that occurred several years ago. We intend to vigorously defend this matter.”
For Barclays, the sanction is the latest of a series of scandals that include a $450 million (343 million euros) fine by US and UK regulators for rigging global benchmark interest rates last year.
But unlike its settlement over Libor (London Interbank Offered Rate), where the bank accepted wrongdoing, it has fought the FERC allegations from the start.