Reform UK is promising to increase the tax-free personal allowance to £15,000, if it wins the next general election.

Robert Jenrick, the party’s economic spokesman, has pledged to introduce the tax cut within the first 100 days if he became chancellor saying it would provide “money that people really need right now”.

It is seen by Reform insiders as the flagship policy announcement of its conference and perhaps their biggest tax policy between now and the general election.

Asked about two senior Reform aides being investigated by the party following a Channel 4 News undercover broadcast into claims about funding, Jenrick told the BBC Nigel Farage had acted swiftly and they had been suspended.

In undercover filming by Verbatim Investigations Farage’s senior aide Dan Jukes and Reform’s head of policy James Orr suggested a way around electoral law to secure foreign party donations for the party, one of which would have totalled £500,000.

Electoral law sets out that to be a donor to a UK political party you must be a UK registered voter, or a company registered in the UK.

Farage said on Friday that his party had “done nothing wrong” and had taken “no illegal money. In fact no money was taken at all.”

Jukes denied wrongdoing and said he was stepping away from politics “in order to clear my name”. Orr told Sky News he would cooperate fully with the party’s internal investigation.

Orr is an associate professor of Philosophy of Religion at Cambridge University’s Faculty of Divinity, and responding to the Channel 4 News and Verbatim report, Cambridge University said it was “looking into these matters”.

Jenrick told the Today programme that the party had taken legal advice and it did not believe it had done anything wrong.

Labour and the Liberal Democrats have reported the party to the Metropolitan Police.

The Electoral Commission has said it is considering “all relevant information, in line with our regulatory remit” and was in contact with the police.

On tax measures, Reform estimates that increasing the tax-free threshold by £2,430 would save most taxpayers £500 a year and 2.9 million people would stop paying income tax altogether.

It says the policy would cost £17.7bn in the first year, rising to £21bn by the fifth year and could be paid for through £80bn of public spending cuts the party plans to make.

Jenrick told BBC Breakfast Reform would pay for the policy by “cutting out the waste”.

Reform has said its spending cuts would include a £50bn reduction in welfare spending.

Half of the £353bn of welfare spending goes on the state pension, but Reform has said it will not cut this proportion.

The bulk of the funding would come from reducing the cost of welfare, Jenrick said, by not paying benefits to people who are not British citizens and helping people who are off work due to their mental health back into jobs.

Jenrick also said the UK would “stop paying foreign aid to rich countries”, adding that “we should be looking after British people first”.

“Yes, we’ll pay for disaster relief but the vast majority of that budget should be spent on the priorities of our own people,” he said.

Reform hopes to raise £10bn by scrapping net-zero programmes and Jenrick said, as the UK only amounted to 1% of the world’s emissions, it was not sensible to be trying to decarbonise faster than other countries as that was “just impoverishing our own people”.

The party hopes to raise £8bn by reducing the number of civil servants and £7.1bn by capping the foreign aid budget.

The policy was announced at Jenrick’s speech on the last day of the party’s conference in Birmingham.

“As chancellor, I will wake each morning with one purpose. To make Britain a better place to be a worker than it was the day before,” he said.

“1.3 million were dragged into paying tax last year alone. Why? Because the tax-free allowance has been stuck at 12 and a half grand since 2021. And Labour has frozen it until 2031.

“That betrayal costs a full-time worker on the minimum wage more than £750 a year. And now for the first time, the freeze means they’re going to be taxing the state pension. What a disgrace.”

He said that if he failed to deliver the tax cut in his first Budget he would resign as chancellor.

“No ifs, no buts, I’ll be gone.”

In its manifesto before the general election in 2024, Reform said it wanted to lift the threshold to £20,000. Jenrick told conference activists this remains his long-term aim.

The personal allowance threshold currently stands at £12,570. Anyone earning more than £100,000 starts to lose their personal allowance. For every £2 earned above the threshold the basic allowance decreases by £1.

Shortly after becoming prime minister, Andy Burnham told the Times that the “frustration” of people in his Makerfield constituency about the frozen allowance had “lodged in my mind”.

He later said that “all of this would be looked at” in the upcoming Budget but that any change would be “difficult given the financial circumstances”.

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