Energy giant E.ON has been fined a record £12 million by industry regulator Ofgem after an investigation into mis-selling.
The energy supplier was found to have caused harm through its extensive poor sales practices between June 2010 and December 2013.
Given the vast number of contracts signed in this period, Ofgem said it was likely a large number of customers were mis-sold to by E.ON.
Ofgem said that E.ON failed to properly train and monitor its staff and those it employed through third party telesales agencies, leading to incorrect information being provided to customers on the doorstep and over the phone.
The energy supplier will pay around £35 to 333,000 customers who are normally recipients of the Warm Home Discount and will to around 465,000 customers informing them of how to get in touch to find out whether they were mis-sold to.
Ofgem senior partner in charge of enforcement Sarah Harrison said: "The time is right to draw a line under past supplier bad behaviour and truly rebuild trust so consumers are put at the heart of the energy market.
"E.ON has today taken a good step by accepting responsibility for its actions and putting proper redress in place."
Additional reporting by PA