The number of households in fuel poverty in the UK rose to 3.5 million in 2006, one million more than in 2005, government figures show.
Fuel poverty is defined as households who spend more than 10% of their income on fuel.
The Unite union said thousands more people are likely to suffer from fuel poverty this winter.
But energy watchdog Consumer Focus says the number of homes currently in fuel poverty is as high as five million.
The figures include around 2.75 million homes classed as "vulnerable" - containing a child, elderly person or someone with a long-term illness.
The number of homes in fuel poverty in England rose from 1.5 million in 2005 to 2.4 million in 2006, including an extra 700,000 vulnerable households.
The government said the rise across the UK was due to consumer energy bills increasing by 22% between 2005 and 2006.
Ministers said gas prices rose by a half from low levels in 2003 to 2006, while electricity prices increased by a third.
Energy companies have raised their tariffs further this year.
Tony Woodley, joint leader of the Unite union, said thousands more people would slip into fuel poverty this winter.
He said: "The government cannot stand back while struggling households chose whether to heat or eat.
"If intervention is on hand to bail out the speculators and spivs who have caused this economic turmoil, then our government should not have to think twice about helping the frail and vulnerable heat their homes."
Labour MP Lindsay Hoyle has put down a Commons motion calling on the government to impose a windfall tax on energy companies.
"The government has got to pursue laws which will take the energy companies on", he said.
"It's unfair, and they totally look like they're operating as a cartel. It seems very strange how one puts up the price, and the others all follow."
The Help the Aged charity estimates one in four pensioners live in fuel poverty, which is three million people.
Special advisor to the charity Mervyn Kohler said fuel poverty was escalating "out of control" and the government's response was "feeble".
He said: "What's needed is a government strategy that combines both short and long-term solutions - crisis payments to help with the here and now and, in the longer term, improvements to the energy efficiency of our housing stock."
Environment Minister Hilary Benn said the government was committed to tackling fuel poverty, but "sharply rising energy price rises have made that goal increasingly difficult".
Last month the government unveiled a £910m package of measures with the big energy companies, aimed at helping people with soaring gas and electricity bills.
It includes half price insulation for all households and a freeze on this year's bills for the poorest families.
Energy Minister Malcolm Wicks said the government was not complacent.
"It is the global demand for energy that is pushing up prices, but that is no comfort to the fuel-poor who need support.
"That is why the prime minister launched the substantial energy efficiency package last month; it is why we have required supply companies to improve social tariffs.
"And it is why winter fuel payments for elderly people will increase substantially later this year.
"It is intolerable that the vulnerable could suffer this winter. The government is not complacent. We need to take action on many fronts."
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