The fine is the largest ever imposed on an energy supplier.

In a statement, Ofgem said that a failure of SSE’s management arrangements enabled sales agents to make “misleading and unsubstantiated statements” about savings to customers.

The regulator 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.

In particular, SSE consistently failed to provide clear and accurate information on prices and potential savings to enable customers to make an informed decision about whether to switch suppliers.

Although SSE terminated doorstep sales in July 2011, failures in telephone and in store sales persisted.

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.

“These failings did not just take place on the doorstep but also in the management of SSE. Ofgem’s fine reflects an absence of effective management control over energy selling.

“Today’s fine sends a clear message to suppliers that Ofgem will hold to account those companies which fail to treat consumers fairly. It is time for the energy industry to take note and get behind Ofgem’s reforms to rebuild trust and make the market simpler, clearer and fairer for consumers.”

SSE has apologised to customers.

In a statement, the firm said: “SSE is deeply regretful that breaches occurred and apologises unreservedly to any customers who have been affected by sales activity which ran counter to the values and culture of the company. It has taken important lessons on board and has transformed its approach to sales to ensure that it will not fall short of the standards all of its customers deserve.”

According to Ofgem, SSE has in place a £5m misselling fund, where customers can receive compensation if they have been missold to. Anyone who believes they have been missold to by SSE should call the fund’s dedicated line on 0845 0707 388.