A third of working families are struggling to pay their energy bills and half are planning to cut their use to afford them, a survey has found.
Almost four million working households (32%) are already under financial pressure over their gas and electricity bills, ahead of widespread predictions that major suppliers will raise their prices, the poll for uSwitch found.
Half of working families (49%) are planning to cut their energy use over the winter to afford their bills and 29% are not switching on the heating even when their house is cold, despite 66% worrying about the effects on their family's health.
More than half of these families (54%) are concerned that cutting their energy use will negatively affect their child's performance at school and 61% say their children do not focus as well on homework when their house is cold.
One in 10 families (10%) have lived in one room in their house, a fifth (19%) have left the oven door open to heat their home and 13% have gone without household essentials to pay for their bills, the survey found.
The poll follows Business Secretary Greg Clark saying he would be calling Energy UK to a meeting to discuss reports suggesting suppliers are overcharging families and making six times the profit they admit to publicly.
The allegations, reported by the Sun newspaper, are based on a report commissioned by Energy UK, which represents power firms, by accountancy firm PWC.
Claire Osborne, energy spokeswoman at uSwitch, said: "It's appalling that even families in work are struggling to pay their energy bills - and we haven't even seen any major price hikes for nearly three years.
"The biggest worry is that, with signs that prices are on the up, many more families could feel forced to keep the heating down in what's set to be a very cold winter.
"When you consider that millions of households are already in debt to their energy supplier, it's a stark warning that consumers feel they have no other option than to ration their energy."
The Competition and Markets Authority (CMA) reported earlier this year that consumers are spending £1.4 billion more than they need to on gas and electricity by remaining on expensive standard variable tariffs.
:: Opinium surveyed 2,000 adults in working families online between October 20-26.
An Ofgem spokesman said: "Ofgem is working to implement the CMA's remedies as quickly and effectively as possible as part of our wider reforms to deliver a fairer, more competitive and smarter energy market.
"We want suppliers to engage more actively with customers, particularly those on standard variable tariffs, to help them get a better deal.
"The large suppliers have already seen their market share fall from 99% to less than 85% in four years. Suppliers who do not engage with their customers to offer them better deals risk losing more customers to rival suppliers who do.
"People who fall behind on their energy bill this winter should contact their supplier who should set up a tailored repayment plan to help them pay off their debts."