Ofgem has opened an investigation into energy supplier E.ON to establish whether it is complying with selling obligations, the regulator announced.
It follows information about E.ON's marketing activities which has "come to the regulator's attention", it said.
The investigation will examine whether E.ON is complying with regulations governing face-to-face and telephone sales activities.
Responding to the announcement, E.ON said it believed it had acted in the best interests of consumers at all times and had "robust" controls in place to ensure responsible selling.
E.ON's director of regulation and energy policy Sara Vaughan said: "We are very disappointed at Ofgem's announcement of its investigation as we have always sought to ensure we have the best and most stringent controls in place.
"In recent months, our procedures have been improved even further as they have been assessed during our Reset Review. We will now work with Ofgem on their investigation and will seek to demonstrate that we have always acted in the best interests of consumers and have the right safeguards in place to ensure our principles are never compromised.
"Our ongoing Reset Review is looking at every aspect of how we support our customers and whilst we will not pre-judge any results that might arise from those discussions, it is clear that a form of face-to-face contact remains an important and desirable way for many consumers - including those who might not otherwise participate in the competitive market - to engage with us."
Ofgem strengthened suppliers' obligations to guard against mis-selling in 2009 and 2010 to ensure that any marketing material is fair, accurate, easy to understand and appropriate for the customer. Suppliers are also required to conduct any telesales and face-to-face marketing activities in a "fair, transparent, appropriate and professional manner".
Ofgem is continuing investigations into the sales practices of Scottish Power, SSE and npower and recently concluded an investigation into EDF Energy on the same issue.
EDF Energy agreed £4.5 million for needy households as part of a settlement with the regulator after staff were found to have made misleading claims to customers.