The two candidates in the Tory leadership contest have ben accused of living in a "fantasy world" as the latest forecasts warned energy bills are set to rise to more than £4,200 a year.
Former Labour leader Ed Miliband criticised Liz Truss and Rishi Sunak, who are vying to become the next prime minister, for failing to outline sufficient support.
There was another hammer blow for Britons on Tuesday when it was forecast that fuel bills will reach £4,266 a year from next January.
Analysts Cornwall Insight said bills are expected to rise even further this winter than previously expected.
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It said energy bills will reach £3,582 per year for the average household from October, up from £1,971 today, and then rise to £4,266 from January and £4,427 from next April.
The dire predictions have heaped renewed pressure on Truss and Sunak, one of whom will be announced as the new Conservative Party leader and prime minister on 5 September.
They are coming in for sustained criticism of their proposals to tackle the UK's current cost of living crisis.
Reacting to the latest energy bills forecast on Tuesday, Miliband tweeted: "As the Tory leadership candidates continue in their fantasy world, this is the catastrophic reality families face this winter.
"Government needs to provide more immediate support, and a plan to insulate homes and roll out renewables to cut bills for good."
Truss has been under fire after she indicated there would be no more "handouts" if she became prime minister, despite warnings that families will face destitution as bills spiral far beyond their means.
She said on Tuesday: “I believe our country’s best days are ahead of us. And what I’m going to do, if selected as prime minister, is keep taxes low, get the economy growing, unleash the potential right across Britain. That’s what I’m about.
“What I am talking about is enabling people to keep more money in their own pockets,” she said.
In a dig at Sunak, the former chancellor, she said: “What I don’t believe in is taxing people to the highest level in 70 years and then giving them their own money back. We are Conservatives. We believe in low taxes.
“What I’m doing is making sure people are paying less taxes and also having a temporary moratorium on the green energy levy to save people money on their fuel bills.
“We will see what the situation is like in the autumn, but I am committed to making sure people are supported and I am committed to growing the economy."
Sunak says that Truss's plan to scrap the increase in National Insurance will do little to help the most vulnerable.
A spokesman for his campaign said: "Liz’s plan will not touch the sides for the majority of British families this winter and pensioners will get no help whatsoever. It seems she is divorced from reality.”
Sunak signalled that he would be prepared to increase direct support for families, warning that they face an “extremely tough winter” ahead.
Defence secretary Ben Wallace, who is backing Truss, said on Tuesday of the predicted energy bill increases: “There’s not a single person who is offering a solution to those significant rises, not the Labour Party, not Rishi Sunak, not anyone else.
“That scale is not entirely going to be solved by the government. I think that’s what people need to understand. That’s the honest truth. We are all feeling it in our pocket.
“And the idea there is a magic wand coming out of Whitehall, no matter who is prime minister, including the Labour Party, is fraudulent to say so.”
The government has has already promised £400 to every household, and extra help for the more vulnerable.
Consumer rights champion Martin Lewis, founder of Money Saving Expert, said: "That rise alone swallows up not just £400 help for all homes, but even the £1,200 for the poorest."
He tweeted: "This will leave many destitute.
“The leadership debate must not ignore this portentous national cataclysm any more.
“They are all in the same party; let’s call on them to come together for the good of the nation rather than personal point-scoring.”
Energy regulator Ofgem said: “The wholesale market continues to move extremely quickly so no forecast for next year is at all robust at this stage and will therefore have very limited value, especially for consumers who must always be the main priority.
“We cannot stop others from making predictions but we would ask that extreme caution is applied to any predictions for the price cap in January or beyond.”
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