Npower has been fined £2m by energy regulator Ofgem for mishandling customer complaints.
It comes months after British Gas was fined £2.5m by the energy regulator over how they handled customers' problems.
Scottish Power and EDF are also being investigated for alleged failings.
On Monday morning Sarah Harrison of Ofgem told BBC Radio 4's Today programme Npower had failed to allow customers access to an ombudsman, warning: "The message is to the companies is they need to get behind better customer service".
Harrison said Npower had resolved the issues which led to the fine, adding: "We have other investigations ongoing into mis-selling practices."