Energy company npower is to pay £3.5 million to vulnerable customers after it was found to have breached energy sales rules.
Regulator Ofgem said the failings of doorstep and telesales staff meant customers were not able to make informed decisions on whether to switch suppliers.
The company remedied the shortcomings by September 2012 but has agreed to make a payment of at least £25 to each of its customers who receive the Warm Home Discount.
Ofgem said npower gained insufficient information about a customer's consumption to enable them to decide whether to switch.
It also failed to ensure that comparisons between the price of npower's supply and that of the customer's current supplier were always based on the tariff that customers were on.
And information on when some consumers would receive their direct debit discount and how direct debit levels would be reviewed was also found to have been inaccurate.