Barclays has been threatened with fines of $470m (£291m) by the US Federal Energy Regulatory Commission (FERC) to settle accusations it sought to manipulate the California energy markets from 2006-2008.
Barclays Bank is under investigation over its integrity less than a year after investigations over its dealings with Arab investors from Qatar were started by the UK government.
Barclays, which has 30 days to contest the fine, denies the charges. This fine is larger than the one it paid over the Libor rate-rigging scandal.
The bank, which has been hit by an interest rigging scandal in the past, is being scrutinized by the department of justice and Securities and Exchange Commission with regard to its relationship with third parties whose compliance with U.S laws is questionable.
Bank officials add that the financial institution is on the brink of being fined by the U.S for its manipulation of power prices leading up to the financial crisis of 2008/9 after investigations revealed foul play.
“We believe that our trading was legitimate and in compliance with applicable law,” Barclays spokesman Mark Lane said. “We have co-operated fully with the FERC investigation, which relates to trading activity that occurred several years ago. We intend to vigorously defend this matter.”