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."
NPower is one of the big six power companies, which dominate the UK gas and electricity markets.
Ofgem said it was also investigating the way EDF Energy handles complaints.
The regulator fined British Gas £2.5m in July for the same offence.
"Energy suppliers now have a golden opportunity to convince consumers that they can be trusted, by getting behind Ofgem's sweeping reforms for the retail market," said Sarah Harrison from Ofgem.
"This is the quickest way of restoring consumer trust in an industry badly tarnished by poor supplier behaviour."
Ofgem introduced new regulations governing the handling of complaints in October 2008, which set out minimum standards that customers should expect if they make a complaint.
They were supposed to complement the work of the Energy Ombudsman, which was established in 2003 to deal with complaints that could not be resolved by the companies themselves.
"It is welcome to see Ofgem willing to hammer home the point that unless energy companies start treating consumers fairly there will be a price to pay," said Adam Scorer from Consumer Focus.
"The energy industry is one of the least trusted by consumers and people need to see real changes to customer service if this trust is to be rebuilt."