Accusing the UK bank for influencing the index price to benefit its financial-swap positions, the FERC also asked Barclays to compensate $34.9m as well as interest for the low-income home energy assistance programs of Arizona, California, Oregon, and Washington.

The four traders accused in the case are Daniel Brin, Scott Connelly, Karen Levine and Ryan Smith, who built and then flattened substantial monthly physical index positions at four most liquid trading points in the Western US.

Charging them for violating the Federal Power Act and FERC’s Anti-Manipulation Rule, the energy market watchdog claimed that all four intentionally coordinated the manipulative plot.

In a statement, FERC said, "Given the seriousness of the violations and the lack of any effort by Barclays and the traders to remedy their violations, FERC ordered Barclays to pay $435 million in penalties."

Protesting against the fine, Barclays said, "We believe the penalty assessed by the FERC is without basis, and we strongly disagree with the allegations."

"We believe our trading was legitimate and in compliance with applicable law," the bank added.

If Barclays and the traders do not pay the penalties to the US Treasury within 30 days determined by FERC, then the regulator may seek affirmation of the penalties from a federal district court.