The energy regulator Ofgem has fined Npower £2m for failing to handle complaints properly.
Ofgem said Npower had not recorded complaints properly or given dissatisfied complainers details of the Energy Ombudsman's redress service.
It added that Npower had now remedied all of the breaches of the regulations for which it was fined.
A spokesman for NPower said: "We are very sorry, we let our high standards slip on this occasion."
"A small number of processes were not correctly adhered to. We have zero tolerance for this type of issue and we'll continue to work hard to make sure our customers are put first."