The UK's so-called "fuel poverty gap" is widening, a government report says, but the total number of people in fuel poverty has fallen.
The Department for Energy and Climate Change (DECC) said there were 4.5 million people in fuel poverty in 2011, a fall of 250,000 on the previous year.
But the fuel poverty gap, a new indicator of the depth of that poverty, went up over the same period.
Any household spending more than 10% of its income on fuel is in fuel poverty.
The figures show that those in fuel poverty would now need an average of £448 more a year, in order to heat their homes properly.
That is an increase of £26 on the previous year.
DECC says that is a result of an increase in energy bills.
It says the total number of people in fuel poverty fell because household incomes rose.
Under a new measure of fuel poverty, known as Low Income High Cost (LIHC), the fall in the numbers would have been more modest.
Under the LIHC measure, 100,000 fewer people were in fuel poverty in 2011, compared with 2010.
The LIHC measure and the fuel poverty gap indicator are being introduced following a review by Professor John Hills of the London School of Economics.
His report into the way that fuel poverty should be measured was published in 2012.