Energy prices will rise for millions of people across the UK in October, right at the start of the cold weather.
Regulator Ofgem said the price cap for default domestic energy deals would be raised to cover suppliers' extra costs.
The typical gas and electricity customer is likely to see their bill go up by £139 to £1,277 a year.
Charities warned the timing would hit struggling families hard, who already face losing an extra £20 a week from Universal Credit in October.
Prepayment customers will see an increase of £153, from £1,156 to £1309, the regulator said.
Ofgem chief executive Jonathan Brearley told the BBC: "The reason the price cap is going up is there has been a record increase in energy prices across the board, not just in gas and electricity but in petrol and diesel."
He urged customers to shop around for the best tariffs, saying there were big savings to be made by switching.
"You don't have to live with this tariff. The price cap is a backstop. We'd encourage any customer, particularly those struggling to pay their bills, to contact their supplier, and get access to a wide-range of help and support," he said.
"This is a devastating increase," said Peter Smith, director of policy and advocacy at fuel poverty charity National Energy Action.
"Millions of household budgets are already stretched to the limit and this massive increase could not be coming at a worse time."
Ofgem said rising wholesale costs were behind the increase, adding that the existence of the price cap meant households would save between £75-£100 a year.
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