Ofgem said the company, the British subsidiary of French energy giant EDF, was not able to properly handle customer complaints between May 2011 and January 2012 when it was introducing a new IT system.

"EDF Energy did not have appropriate procedures in place to properly receive, record and process all customers' complaints in accordance with complaints handling rules," said Ofgem in a statement.

EDF Energy will pay the penalty to two charities that support energy users struggling to pay their bills.

(Reporting by Karolin Schaps; Editing by Li-mei Hoang)