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.
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