Energy privateers had their feet held to the fire on Monday by campaigners and a think tank who slammed them for ripping off households.
As Greenpeace activists blocked British Gas owner Centrica's headquarters, experts at the Institute for Public Policy Research (IPPR) said the gap between the "true cost" of energy and what people were paying would reach £1.9 billion by 2020.
The IPPR said watchdog Ofgem needed to boost competition and make sure prices are fair.
It claimed there was no indication the "big six" firms were passing on lower costs to their customers, instead ripping them off to subsidise special offers.
"The big six are functioning as an oligopoly which ultimately produces negative outcomes for consumers by restricting competition."
Meanwhile in Windsor, Greenpeace activists blockaded "big six" firm Centrica with a giant energy bill. It said average prices had soared £150 in the last year and £100 of that was down to rising gas prices.
And official files dug up by Greenpeace suggest more pain is in store for families as gas supplies shrink and prices rocket.
Centrica, the dirtiest "big six" firm, has been urging the government to burn more gas to line its pockets.
Five activists were inside the building aiming to redecorate millionaire boss Sam Laidlaw's office with easy-peel wallpaper printed with energy bills.
Greenpeace campaigner Lawrence Carter said: "If Centrica instead invested in clean, cutting-edge renewable energy and energy efficiency, they would help to both bring household bills under control and to tackle climate change."
Communist Party general secretary and Cardiff election candidate Rob Griffiths welcomed the report but said campaigners must go further.
He called for public ownership of gas, oil and electricity firms, adding: "What domestic and business customers need urgently are price controls with a windfall tax on the monopoly profits of the big energy companies."
"And the big chiefs at regulator Ofgem should be sacked because their apologies and excuses for big business profiteering have completely run out of time and lack any credibility whatsoever."
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