Energy regulator Ofgem has fined Npower a record £26mln for misbilling and treating customers badly as the state of Britain's energy supply business continues to take a battering.
It is the largest settlement yet agreed between Ofgem and a so-called "big six" energy supplier- a group that includes British gas owner Centrica (LON:CNA) and SSE (LON:SSE).
Ofgem revealed the record sum would be divided between some of the worst affected customers and charity.
It has emerged that between September 2013 and December 2014, Npower issued more than 500,000 late bills, while some were also inaccurate.
In addition, a staggering 2mln complaints were received from Npower customers over the 15 month period.
The watchdog however did concede that many of the group's problems had happened after it introduced a new IT system in 2011 and the firm was fined £2m in October that year for a similar offence.
It is not the first such computer glitch problem that has affected the sector.
In October this year, British Gas emailed more than 2,000 of its customers to warn that somehow their e-mails and passwords had ended up online.
The big six utilities dominate the UK's energy landscape and still control more than 85% of the market but customers have begun to fight back and turn to smaller and more regional firms.
Dermot Nolan, Ofgem chief executive, was reported as saying: "Npower failed its customers. Not only have its billing and complaint handling procedures been chaotic, it treated many of its customers poorly, which is completely unacceptable.
"Npower's management failed to act quickly enough to protect its customers when things went wrong with changes to its IT system."
NPower said it was sorry about what has happened.
It said it had reduced complaints by nearly 70% since the beginning of this year.
SSE nudged up 0.2% today to 1,486p and Centrica added 0.33% to 214.2p.