Labour will on Monday call for the energy price cap to be frozen at its current level of £1,971 as UK households struggle to pay their bills.
The move to block an expected increase to £3,300 in October is expected to put further pressure on the two Tory leadership contenders.
A think-tank has warned households on low incomes will have to reduce their spending power by three times as much as high income households in order to afford their energy bills this winter.
Labour leader Sir Keir Starmer will call for the price-cap freeze when he sets out details of his party’s plans, including how they will pay for the measure, according to The Observer.
Specifics of the measure were not available, though Sir Keir wrote elsewhere that the party aimed to end energy “injustice”.
“We would end the injustice that sees people on prepayment meters paying over the odds for their energy,” he wrote in The Sunday Mirror.
“And we will set out how we would help people directly this winter in the coming days.”
It comes after Sir Keir on Friday said it is “nonsense” to claim his party has not been leading on the cost-of-living crisis.
Last week the party announced it wanted to put a stop to “outrageous” premiums that energy prepayment meter customers face.
Shadow chancellor Rachel Reeves said Labour would end the “unjustifiable” practice that can result in people with energy prepayment meters being charged more than those who pay by direct debit.