Millions of households face an unprecedented 80% increase in their energy costs in October, taking a typical bill to £3,549 a year.
Regulator Ofgem's new energy price cap means a home in England, Wales and Scotland using a typical amount of energy will pay nearly £300 a month.
Charities warn some people may not be able to afford both heating and food.
Ofgem boss Jonathan Brearley admitted that the rise would be "devastating for many families".
He said that the new prime minister - who will be named on 5 September when the Conservative Party chooses a new leader - will "need to act urgently and decisively to address this".
He told the BBC: "All of us, with the government, need to get ahead of this problem and manage it on behalf of customers because the outlook for winter without any action looks very difficult indeed."
So far the government has promised a £15bn package of support for UK households. This includes two payments totalling £650 for eight million low-income households on benefits and a £400 grant for all homes.
A typical annual bill is currently £1,971. New forecasts suggest that could peak at well over £6,000 a year next year, heaping pressure on a new prime minister to give urgent extra support for those struggling or unable to pay.
Prepayment meter customers, often among the most vulnerable consumers, will pay an extra £59 a year on their bill from October, taking the new typical bill on a prepayment meter to £3,608.
Consumers' bills are rising owing to soaring wholesale costs faced by suppliers. Energy prices rose sharply when lockdown was lifted and the economy returned to normal.
They have also increased because the war in Ukraine has reduced supplies of Russian gas.
The new cap is in effect from October to January, after which it is expected to rise further.
While this rise in bills is huge, there could be another massive increase in January, and then again in the spring.
Analysts at consultancy Cornwall Insight, who have previously given accurate forecasts, suggest that a typical annual bill could hit nearly £5,400 in January, and £6,600 in April.
"While there is still some time until the January and April caps are set, the energy crisis is showing no sign of abating," said senior consultant Craig Lowery.
Businesses are not covered by a cap and many face cost pressures that could be passed on to consumers in higher prices.
Prices have also been rising in Northern Ireland, where there is no cap owing to the limited number of suppliers.
What is the energy price cap?
The cap is a limit on how much domestic customers in England, Wales and Scotland pay for each unit of gas and electricity, and on how much they can be charged for being connected to the grid (the standing charge).
It is set by the regulator Ofgem based on how much energy firms are having to pay for gas and electricity and will next be changed in October. It applies only to providers' standard and default tariffs, which most people are on. There is a separate cap for prepayment meter customers.
Up until now, a cap has been in place for six months, but from October it will be reviewed and changed every three months.
Chancellor Nadhim Zahawi said: "I know this will cause stress and anxiety for many people, but help is coming with £400 off energy bills for all, the second instalment of a £650 payment for vulnerable households, and £300 for all pensioners.
He added: "I am working flat out to develop options for further support. This will mean the incoming prime minister can hit the ground running and deliver support to those who need it most, as soon as possible".
The rise in the cap in England, Wales and Scotland will put considerable strain on low-income households, according to charities.
'I'm living on nothing'
Among those already struggling with their finances is Marina Keohane, from Gainsborough in Ipswich, who said the thought of higher fuel bills this winter brings tears to her eyes.
"To actually fear every day how you're meant to live, that's just something else. I'm living on nothing," she said.
"I don't use anything, the only thing that's on all the time is the fridge freezer. I have a TV on, I don't have the light on - I sit in darkness.
"Eating wise, I'm scared to use my cooker. It's electric and obviously if you put something in the oven it uses more [energy] so I try to stick to ready meals in the microwave or do salads."
Government support
No 10 has previously said there would be no new support offered before a new prime minister is appointed in early September.
The current favourite to replace Boris Johnson, Liz Truss, has promised tax cuts and recently hinted at direct financial help for hard-pressed households.
The foreign secretary, writing in the Daily Mail, said she would take "decisive action on entering No 10 to provide immediate support".
Ms Truss said she would help people who are "feeling the squeeze" get through tough times, and repeated her pledge to hold an emergency budget.
However, she said it was not "right" to announce her full plans before the leadership race is over.
Her rival, Rishi Sunak, has said he would introduce more targeted support for households, and has promised to reduce VAT on domestic energy bills from 5% to zero.
Labour wants the energy price cap to be frozen at its current level and paid for, in part, by a big increase in tax on oil and gas company profits.
Leading figures in the energy sector have called on the government to introduce an Energy Tariff Deficit Fund to spread out the cost for households over a decade, assuming prices eventually fall.
Source: BBC
Image source: Pixabay
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