The company's thermal generation unit (EDF ETG) "regularly sent misleading signals to the National Grid electricity system operator (ESO) about the capabilities of its generation plant" from September 2017 to March 2020, Ofgem monitoring had found.

EDF Energy regularly inflated the minimum amount of power - known as the stable export limit (SEL) - that it said its West Burton B (WBB) plant could supply at times it did not plan to generate electricity, it added.

"In many cases, this meant that the ESO had to purchase more energy from the plant than needed, when the plant was used to balance the system," Ofgem said.

EDF ETG had admitted it breached its obligations under Britain's grid code and the Regulation on Energy Market Integrity and Transparency (REMIT) and agreed to make a voluntary payment of 6 million pounds, Ofgem added.

"Ofgem has concluded that there is no merit in opening a formal investigation," the regulator said.

"Although the breach was inadvertent, and EDF ETG considered its approach would reduce costs for the electricity system operator, we should have done better," the company said in a statement, adding that it took the issue "extremely seriously and apologises for the error".

It had now changed its approach to ensure it fully complied with regulations.

($1 = 0.7429 pounds)

(Reporting by Nora Buli in Oslo; Editing by David Goodman)