E.Ons Chief Executive, Tony Cocker, said on Wednesday the energy supplier could not guarantee it would not raise prices in the UK this year following a hike in January.
The company was the last of the big six energy giants to increase its tariffs at the start of the year and has been fighting claims of over-charging customers ever since.
E.On had blamed higher wholesale prices and greater investment commitments for higher bills.
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Cocker told the BBC's Radio 4 Today programme: "We can't guarantee we won't have to raise prices again this year."
His statement came after the company reported a 17% fall in 2012 profits to £235m.
Profit margin electricity and gas to domestic customers in the UK was estimated at 2.3%, or just under £27 on an average weighted dual fuel residential bill.
The company's sales revenues rose 5.0% to £132.1bn.
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