UK energy regulator Ofgem has today fined utility SSE £10.5m ($15.8m) – the largest financial penalty ever imposed on an energy supplier.

The fine relates to sales tactics carried out by SSE in telephone, in-store and doorstep transactions and the size of the penalty “reflects the seriousness and duration of breaches, the likely substantial harm that they have caused and the likely gain to SSE”, said Ofgem in a statement.

UK Energy Minister Michael Fallon said today that he had “rarely seen a worse case of consumers being misled so badly”.

SSE – formerly Scottish and Southern Energy – is one of Britain’s so-called ‘Big Six’ energy companies are operates coal and gas plants as well as on and offshore wind farms and biomass and hydro plants.

Ofgem said it “found failings at all stages of SSE’s sales processes, from the opening lines on the doorstep, in-store or over the phone through to the confirmation process which follows a sale”.

It added that 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, missold to them and undermined trust in the energy supply industry.

She said that today’s fine “sends a clear message to suppliers that Ofgem will hold to account those companies which fail to treat consumers fairly”.

Examples of SSE’s misdemeanors included telling customers they would save money when in fact they were switched to a more expensive contract; that other suppliers were making “all sorts of false promises” and putting their prices up, and that other suppliers’ price increases were higher than they actually were.

SSE corporate affairs director Alan Young told the BBC this morning that 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. We have totally reformed our business in this area.”