Ofgem has revealed that it is investigating whether members of E.ON's staff have used misleading sales information in a bid to lure customers away from its rivals.
The industry regulator is investigating both sales made over the telephone and on the doorstep, but did not reveal the nature of the evidence it had acted on or how it had been obtained.
E.ON has responded by stating that its sales staff had adhered to the rules.
Sara Vaughan, of E.ON, commented on Ofgem's decision.
She said: "In recent months our procedures have been improved even further.
"We will now work with Ofgem on their investigation and will demonstrate that we have always acted in the best interests of consumers," she added.
Consumer Focus stated that mis-selling had been a regular problem within the energy industry and needed to be resolved "head on" by Ofgem.
Director of Energy at Consumer Focus, Audrey Gallacher, commented: "While it is unclear what types of sales E.ON may be suspected of misleading customers on, we would hope this announcement will cause them to rethink ending doorstop sales."
"E.ON has persistently failed to take action on this unwanted sales activity, despite being the only major supplier left selling at people's home," Gallacher concluded.
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