Reform UK would raise the tax-free personal allowance to £15,000 in the first 100 days of the party’s government, Robert Jenrick has vowed.
The party’s Treasury spokesman pledged to resign if he failed to deliver the promise, which would benefit 40 million people.
The personal allowance - the amount a worker can earn before they begin paying income tax - has been frozen at £12,570 since 2021.
It is due to be extended until 2031 under current Treasury plans.
Mr Jenrick told Reform’s party conference in Birmingham on Saturday: “Here is my pledge to you and to the country.
“In the first 100 days of a Reform government, in our first Reform budget, we will raise the tax-free personal allowance from £12,570, not just with inflation, not just to £13,000, not just to £14,000, but to £15,000.
“The first 15 grand you earn is yours. Every penny, every penny is yours tax-free, £500 a year for the average basic rate taxpayer.”
He said: “A tax cut for 40 million people. The biggest rise in the personal allowance in its history, and delivered not in 10 years but in my very first budget.”
Mr Jenrick added: “If I don’t deliver this exactly when I have said that I will, I will resign. No ifs, not buts, I will be gone.”
Labour accused Reform of using the announcement to distract from the funding scandals engulfing leader Nigel Farage.
The party is facing a police investigation over allegations it breached political donation rules by having polling paid for by an overseas company.
A Labour Party spokesperson said: "Robert Jenrick only seems to be interested in making people better off when Reform UK are trying to distract from the latest scandal engulfing their party.
“The public will see right through it.
"Farage and Reform are funding policy after policy through mystery spending cuts, which could put local schools, hospitals and other vital public services at risk.
"Meanwhile as soon as Andy Burnham became Prime Minister, he prioritised cutting taxes to provide much needed breathing space for families and businesses on the cost of living.”
Mr Jenrick also hit out at John Healey during his keynote conference speech, branding the Chancellor the “biggest fraud” in the Cabinet.
“He’s a man who resigned on principle because the Treasury wouldn’t fund defence, he now runs the Treasury,” Mr Jenrick said.
“He is the Chancellor of the Exchequer, and what has Chancellor Healey offered our armed forces? Nothing.
“The very same deal he resigned over. What a price his principles fetched, a promotion, a new red box, a grace and favour flat in Downing Street, and a total betrayal of everything he claimed to stand for.”
