Sir Keir Starmer has warned that even middle-income earners could struggle to pay their energy bills this winter.
Britons face double-digit inflation and an economy heading into a potentially lengthy recession, while the increase of the energy price cap will see the typical household bill rise to £3,549.
Charities have warned that the poorest members of society will suffer the most during the cost-of-living crisis but Starmer has warned that soaring energy bills would leave people earning as much as £50,000 a year under pressure.
The Labour leader told Channel 5’s Jeremy Vine: “I think if you’re on £50 grand you’re going to really struggle with £4,000 on your energy bills.
“I think there will be many people watching who accept ‘I’m not the hardest-up, I’ve got a decent wage, but £4,000 on my energy bills is more than I can afford’.”
The government previously announced that millions of households across the country will receive a £400 discount on their energy bills this winter in an attempt to alleviate the crisis.
But Labour have called for a complete freeze on energy prices, funded in part by a windfall tax on oil and gas giants.
The freeze, which would last until at March at least and cost around £29bn, would apply to all households, regardless of income, which Labour says would save them an average of £1,000 this winter.
Watch: Labour leader accuses government of being 'missing in action'
The loss to energy retailers would be funded by Labour to stop the firms going bust.
Labour’s plan comes after Ofgem confirmed last week that energy prices for the 24 million British households on the energy price cap will rise by 80% in October from £1,971 to £3,549.
The figure could rise to around £5,400 at the beginning of January.
The Resolution Foundation think-tank said the country is facing a “winter catastrophe” unless the government provides extra support.
It said any extra support needs to target those who need help most, while also reflecting households’ differing level of energy usage and stretching beyond simply the benefit system.
Financial expert Martin Lewis has also warned of an impending “catastrophe” of huge price rises over the next few months.
He told LBC: "I have catastrophised this because it is a catastrophe.
“I'm now begging and pleading with whomever the new prime minister is. It should have been dealt with far sooner.”
Either Liz Truss or Rishi Sunak will become the new prime minister next week but neither candidate has so far revealed what they intend to do to ease pressure on millions over the winter months.
Truss, who is the favourite to take over from Boris Johnson on Monday, is expected to deliver an emergency budget in the coming weeks but critics have argued that measures should be announced now as the situation is “urgent”.
Lewis added: "I am here to beg, to plead, to pray that our new prime minister understands how urgent this is and, while we've left it late, on 5 September, on that day, lays out concrete plans that will stop a sizeable chunk of our population having genuine health risks because of lack of food and heat due to this horrible expense, and another sizeable chunk of lower and middle earners from having the catastrophic impact on their standard of living."
Truss is understood to be mulling over tax cuts to help the most vulnerable but her plan would crash the economy, according to a leading economist.
Institute for Fiscal Studies boss Paul Johnson said Truss’ plans were "simplistic" and "quite worrying".
He told The Times: "You clearly can’t do all of this without completely crashing the public finances.
“This simplistic mantra that you cut taxes and the economy grows more, that you cut taxes when you have a big deficit and high inflation, and you don’t do it with any other part of the plan, is quite worrying.”
A government spokesperson highlighted their £37bn package of help for households, including £1,200 extra support for one in four households, as well as the £400 discount on energy bills for all households.