The “Big Six” energy giants have been accused of overcharging customers by nearly £4billion over just three years.
Labour claimed the firms, which supply 98% of homes, profited by buying wholesale electricity from their own power plants at above the market price.
They then passed the cost on to customers, who each paid an average of £150 over the odds, according to Shadow Energy Secretary Caroline Flint.
She fumed: “These figures reveal the full extent of the way consumers have been overcharged for electricity.
“Energy companies always blame wholesale costs when they put up bills.
“But now it looks like they could have deliberately inflated prices to boost profits from their power stations.”
Labour looked at the prices paid for wholesale electricity by British Gas, EDF, npower, Scottish Power, E.on and SSE between 2010 and 2012.
They compared it with the market price paid by small energy supplier First Utility. The difference was £3.9billion.
But industry body Energy UK, which represents the major power firms, claimed Ms Flint got her maths wrong.
It said the price its clients paid – known as the weighted average cost of fuel – included extra supply-related costs above the basic market price.
Big Six energy firms 'overcharged customers by £4 BILLION over three years'