The cost of energy for British consumers will rise by 80% from October, the UK's regulator said on Friday. Ofgem's move will see average annual household bills soar to £3,549 (€4,204).
The move looks set to further fuel inflation amid a cost of living crisis and put pressure on the government to act.
Ofgem Chief Executive Jonathan Brearley said the rise would have a "massive impact" on households across Britain, and another increase was likely in January, reflecting significant pricing pressure in energy markets.
He said the government needed to deliver further help to households.
"The government support package is delivering help right now, but it’s clear the new Prime Minister will need to act further to tackle the impact of the price rises that are coming in October and next year," he said.
"The response will need to match the scale of the crisis we have before us."
Finance minister Nadhim Zahawi said he was working on a plan to be ready for the next government, which will be appointed when either Liz Truss or Rishi Sunak becomes prime minister on 5 September.
The regulator's move comes amid soaring global gas prices, exacerbated since the start of Russia's war in Ukraine which has seen Western sanctions imposed on Moscow.
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