UFC said in a study that when wholesale power prices are low, EDF systematically underutilised its nuclear plants, which leads to more expensive fossil-fuel fired power generation assets to be used, and in turn results in higher power prices.

The consumer group said it estimated EDF had boosted its revenue by about 2.3 billion euros (2.00 billion pounds) in this way between 2012 and 2017.

EDF denied any wrongdoing.

"EDF at all times optimises its power generation fleet ... and operates under the close supervision of market regulator CRE," EDF said in a statement.

The CRE was not immediately available for comment.

(Reporting by Geert De Clercq; Editing by Sudip Kar-Gupta)