Co-operative Energy has been ordered to return £1.8m to customers after repeated customer service failings.
The firm blamed the problems, thought to have affected a significant chunk of its customers, on a new billing system, which went live last year. Since then, the energy supplier has been beset with technical issues and customer complaint levels have soared.
Ofgem said Co-operative energy has given assurances that the problem has been addressed and that customers would receive adequate service.
The energy supplier has already paid affected customers £1.6m in compensation.