Energy watchdog Ofgem has today fined SSE £10.5 million for the "prolonged" mis-selling of gas and electricity.
It is the largest fine ever imposed on an energy supplier.
In a statement, Ofgem said the energy giant, SSE, formerly known as Scottish and Southern Energy, was guilty of failings at all stage of the selling process, "from the opening lines on the doorstep, in-store or over the phone through to the confirmation process which follows a sale".
It also said: "The level of fine reflects the seriousness and duration of breaches, the likely substantial harm that they have caused and the likely gain to SSE.
"In particular, SSE consistently failed, over a prolonged period of time, to conduct its sales activities in a way that would provide clear and accurate information on prices and potential savings to enable customers to make an informed decision about whether to switch suppliers."
Sarah Harrison, Ofgem's senior partner in charge of enforcement, said: "In order to restore trust in the energy market suppliers must comply with their obligations and play it straight with consumers.
"Ofgem's findings show SSE failed its customers, mis-sold to them and undermined trust in the energy supply industry.
"Today's fine sends a clear message to suppliers that Ofgem will hold to account those companies which fail to treat consumers fairly.
George Charles, marketing director of vouchercodespro.co.uk, said: "We can already see the SSE fine is having an immediately damaging impact on their customer base, as we have had unprecedented levels of people searching for energy supplier savings since the fine hit the headlines.
"It is clear that consumers are just not willing to tolerate such shocking money making tactics from energy suppliers at a time when money is so tight."
Alan Young, SSE's corporate affairs director, told the BBC the company was "very sorry". He added: "What we were doing was not adequately telling people about the terms and conditions of their contract or adequately making sure they had the information they needed to switch."
SSE has set aside a £5 million mis-selling fund to compensate its customers. Ofgem is encouraging anyone who believes they have been mis-sold to by SSE to call the fund's dedicated phoneline on 0845 0707 388.