Energy companies have insisted controversial rises in bills were partly down to green taxes but were accused of charging customers "the maximum price they feel they can get away with".
Senior executives from the "Big Six" - E.ON, British Gas, npower, EDF, Scottish Power and SSE - have been questioned by MPs about the recent increases by some of them.
The Energy and Climate Change Committee hearing was told rising wholesale costs and environmental "stealth taxes" were behind the average 9.1% hike.
But the businesses practices of the Big Six were called into question by the managing director of small, green energy firm Ovo Energy, who also faced the politicians.
Stephen Fitzpatrick told MPs he "can't explain" the price rises being imposed because his company was buying gas at a cheaper price - 5p a therm less - than it had in 2009.
Loyal bill-payers are charged a far higher rate, in some cases £200 more, and loaded with environmental costs than those who switch but the companies responsible go "unchallenged" by Ofgem, he said.
He said: "It looks to me like a lot of energy companies, a significant number of the Big Six, are charging the maximum price they feel they can get away with to the customers that they feel will not switch under any circumstances."
And he claimed if the Big Six charged the same unit rates as Ovo Energy did in 2012, then the total saving would be £3.7bn (£1.4bn in gas and £2.3bn in electricity).
Tony Cocker, chief executive of E.ON, which has yet to announce a price increase, said he had written to Prime Minister David Cameron calling for a competition commission inquiry to investigate the industry to help reassure customers.
Npower said it supported the call for an inquiry but Centrica (British Gas) and SSE said they were against.
An analysis by industry regulator Ofgem showed that wholesale prices have risen by 1.7% - adding just £10 to the average household bill of £600.
But npower's external affairs director Guy Johnson said that wholesale prices had increased by 3% and that wholesale prices accounted for 45% of their costs.
He also blamed the 11.1% increase on household bills announced last week by npower on green levies, saying they had increased by 31% and now accounted for 15% of the companies' costs.
In addition, he said that transport costs, which comprised 23% of their costs, had also increased by 10%.
When asked why, over the last few years, the Big Six had all increased prices at a similar level and at a similar time, they claimed it was because the majority of their costs were outside their control and therefore broadly the same.
William Morris, retail managing director of SSE, whose announcement of a 8.2% increase sparked the latest round of price rises, said that his company had been first to announce hikes at each time in the last two years.
He said that "85% of our costs are outside our control" and said that because of this "they are going to be similar at the same time".
Mr Cocker said that his firm E.ON would "hold out as long as we can" over a possible price rise.
However, he said the drivers for each firm to make the increases were similar, citing increases in transport and wholesale costs, and "environmental obligations" (green taxes).
Mr Cocker said that the green levies were a "stealth poll tax" and should be removed from energy bills and put into general taxation - a move the Liberal Democrats have indicated they may not oppose.
Mr Morris, of SSE, said that if the Government was to agree then that would "come straight off the customers' bills".
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