Tory leadership frontrunner Liz Truss says she will immediately reverse the National Insurance rise if she wins the keys to No10.
Conservative members will elect Boris Johnson's successor, with the new Prime Minister announced on September 5.
The Foreign Secretary has pledged to scrap the tax that was brought in by former Chancellor and leadership rival Rishi Sunak, throughout her campaign.
Her campaign team initially thought they would only be able to cut the tax in April 2023. But they now believe it can be reversed within weeks.
Penny Mordaunt this morning said she publicly backed Ms Truss's campaign because "she understands our nation is at an inflection point".
Ms Truss and Mr Sunak have been facing mounting pressure to set out plans to tackle the deepening cost of living crisis.
The pair have also been clashing continued to clash over their plans for the economy amid warnings of a looming recession and inflation set to soar to more than 13%.
Former prime minister Gordon Brown last night demanded the Government come up with an emergency budget before a “financial timebomb” in October “pushes millions over the edge”.
The Foreign Secretary has rejected the idea of giving “handouts” for struggling Brits, promising to cut taxes instead.
Writing for The Sunday Telegraph, Ms Truss said that she would use the budget to “immediately tackle the cost of living crisis by cutting taxes, reversing the rise on National Insurance and suspending the green levy on energy bills ”.
Suggesting the possibility of rolling out one-off support for struggling households, Ms Truss added: "I will look at what more can be done, but the way I would do things is in a Conservative way.
"We would put more money back in the pockets of hard-working people without delay. That is necessary, affordable and the right thing to do at a time when we face the highest tax burden in 70 years.”
Ms Truss has also vowed to keep corporation tax at 19 per cent and set up new low-tax, low-regulation investment zones around the country.
Mr Sunak has said inflation must be brought under control before taxes are lowered. He blasted Ms Truss’s promised unfunded tax cuts in an interview with The Times.
The former chancellor said: “The priority for me is to not do things that make it worse and I think putting £40 billion plus and borrowed money into an economy that’s seeing an inflation spiral does risk making it worse.
“It might be okay but I think it means taking a gamble with people’s savings, their pensions, their mortgage rates, it’s not a gamble I’m prepared to take so I don’t want to make it worse.”
Mr Brown said the Prime Minister Boris Johnson, Rishi Sunak and Liz Truss must agree to emergency measures “this week”.
Writing in The Observer, he said: “A financial timebomb will explode for families in October as a second round of fuel price rises in six months sends shockwaves through every household and pushes millions over the edge."
His warning came after a new report commissioned by Mr Brown suggested Government help has failed to address households’ needs.
A government spokesperson said they understood people were struggling with rising prices and that was why it had made the extra £1,200 available to eight million of the most vulnerable households.
During a campaign visit to Solihull on Saturday, the Foreign Secretary told reporters: “What I’m about as a Conservative is people keeping more of their own money, growing the economy so we avoid a recession and the best way to do that is lower taxes, but also unleashing investment into our economy.”
Former Tory leadership contender Penny Mordaunt told Sk News Ms Truss's tax cuts are "funded" and "she has a plan to get growth back into our economy".
She hit out at Mr Sunak’s economic legacy as chancellor, saying: “Under the plans at present, what we know is Britain is headed for a recession.
“That is not inevitable, but we need to avoid that by making sure our economy is competitive, that we’re encouraging businesses to grow and that we are keeping taxes low."
