EDF Energy has acknowledged that its customers were caused significant disruption when the firm introduced a new IT system in 2011 and has publicly apologised for this.
Ofgem said that the firm's payment of £3 million to vulnerable customers was a "step in the right direction".
EDF Energy failed to have sufficiently robust processes in place when they introduced a new IT system and this led to the unacceptable handling of complaints.
Their commitment to putting things right and paying £3 million to the Citizens Advice Energy Best Deal Extra scheme and the Plymouth Citizen Advice Bureau's Debt Helpline to benefit vulnerable customers is a step in the right direction to rebuilding consumer trust.
More top news
Live ITV News coverage after the explosion at Manchester Arena following a pop concert by US singer Ariana Grande.
What separates us from the terrorists is that we air our grievances with non-violent debate though our democratic structures and processes.
A York College student has written an appeal on Facebook to help find her parents who have gone missing after the Manchester terror attack.