Sir Keir Starmer will lay out his party's "comprehensive plan" to deal with the costs of living crisis - including rising energy bills - on Monday.
Labour on Thursday vowed to stop the "outrageous" premiums that energy prepayment meter customers face.
It comes as Boris Johnson doubled down on his insistence that it is for his successor to "make significant fiscal decisions" after talks with energy bosses ended with no new measures to ease the cost of living crisis.
Speaking on Friday, Sir Keir said he would outline his new policy proposal in more detail next week.
"We do need a strategic, a credible, plan and that's exactly what's missing from this government," he said.
He added: "On Monday I'm going to be setting out a comprehensive set of proposals, a plan for how we handle the upcoming costs in the autumn, while what you've had from the Conservative Party is two leadership candidates arguing with each other about just how appalling their record in government has been, and a prime minister who's a lame duck - he recognises there's a problem and he's not prepared to do anything about it."
The Labour leader highlighted that his party proposed a windfall tax on energy companies, which the government then introduced. He also said his party had proposed taking VAT off energy bills - something Rishi Sunak was now advocating for.
Sir Keir said: "For the best part of 12 months, Labour has been absolutely leading on this issue."
Earlier on shadow justice secretary Steve Reed denied that Gordon Brown was leading the party's policy in Sir Keir's absence after the ex-PM called for energy firms to be temporarily nationalised, in his third major intervention this week.
Meanwhile, Mr Johnson yesterday said he would continue to urge the energy sector to ease the financial pressures facing struggling families, but he repeated his stance that it is for his successor in Number 10, either Liz Truss or Rishi Sunak, to "make significant fiscal decisions", a Treasury spokesperson said.
Energy analysts have predicted that typical energy bills could rise to approximately £3,500 in October and more than £4,200 in January.
Mr Johnson's successor will not be announced until 5 September.
Shadow chancellor Rachel Reeves has now confirmed that part of Labour's action would be to end the "unjustifiable" cost of prepayment energy meters.
Specifically, the use of premiums that can result in people with energy prepayment meters being charged more than those who pay by direct debit.
The party says the measure is part of a wider cost of living package being developed by Sir Keir, who, along with Mr Johnson, has been criticised in the last week for taking a holiday at a time of crisis.
Labour says the prepayment meter policy would help to bring prepayment energy prices into line with those for direct debit customers and estimates that it would provide relief to four million households.
"It's outrageous that people on prepayment meters have to pay more for their energy. Why should those with the least have to pay more to heat their homes and put the lights on? This is unjustifiable and morally wrong," Ms Reeves said.
"As energy prices spiral, this unfair prepayment premium must end. Labour would make sure that no-one pays over the odds for the same gas and electricity that everyone else gets, as well as taking broader action to help people manage their bills over the winter."
It is thought that Labour would remove the gap between the two price caps and reimburse energy companies for the difference between October and March - at an estimated cost of £113m.
Labour says this would be paid for by a strengthened windfall tax on oil and gas companies.
A spokesperson for the Department for Business, Energy and Industrial Strategy said: "Pre-payments meters remain an effective way for people to pay for their energy use whilst managing costs and debt, while the energy price cap protects four million pre-payment meter customers from overcharging by energy suppliers.
"We understand that global inflationary pressures are squeezing household finances. This is why we are providing a £37bn package of support to help households in these challenging times, including a £400 discount on energy bills which can be accessed by those on prepayment meters and £1,200 to around eight million low-income households."