Many households are reporting higher energy bills than last year despite one of the mildest winters in a decade, consumer groups have said.
Research for Consumer Focus and Citizens Advice found four in 10 people (40%) are facing higher winter bills despite the warmer weather and resulting fall in energy consumption.
Almost a third of households overall (31%) said their bill was higher than expected, but this figure rose to more than half among those who pay by cash, cheque or prepayment meter, with many in this group on low incomes. Most energy firms' price cuts did not come into effect until late winter.
So at the start of January this year the average annual amount of energy cost £180 more than the year before, according to the two groups. They warned that the unexpectedly high bills could push more people into debt with their energy supplier, particularly those on low fixed incomes such as older people and families on benefits.
Almost half of consumers (48%) already planned to make cut-backs to afford their energy bill, rising to six in 10 (60%) among those with prepayment meters. Of those cutting back, almost half (44%) planned to reduce the amount of energy they use and a third (34%) would cut back on seeing friends and family, holidays and goods such as clothes and music.
A further one in seven of all consumers (13%) planned to cut back on food, with about one in six of the poorest consumers (17%) having to cut back on eating to afford their heating. The poll also revealed that a third of people (33%) could not afford to pay if their monthly bill increased by £30 and more than half (52%) could not afford to pay if their bill rose by £50.
Consumer Focus chief executive Mike O'Connor said: "With energy price cuts and less heating used because of the mild weather, most people will have expected their winter energy bill to be lower this year. So higher winter bills may come as a nasty shock that many customers simply can't afford.
"We'd urge anyone struggling to pay their bill to contact their energy firm and Citizens Advice as soon as possible. The sooner you get in touch the less chance there is for debts to build and become harder to repay."
Citizens Advice chief executive Gillian Guy said: "High energy bills are stretching some household budgets to breaking point. If you are worried you can't afford to pay your bills and they are mounting up, don't suffer in silence. Speak to your energy supplier as they have a duty to make sure you are repaying what you can afford."
TNS Consumer Omnibus questioned 1,295 adults across Britain in March 2012.