Boris Johnson says workers will not have to pay National Insurance until they earn £12,000 if the Conservatives are elected to power.
Answering questions in Teesside, the prime minister promised his party would ensure “low tax for working people”.
The current threshold sees workers paying National Insurance contributions once they earn £8,628 a year.
Mr Johnson had promised to raise the threshold to £12,500 during the Tory leadership contest.
That pledge could see workers saving up to £465 a year.
The Institute for Fiscal Studies had estimated the proposed rise to £12,500 – which would would match the threshold where workers start paying income tax – would cost £11bn.
On a visit to an engineering plant, Mr Johnson was asked by one of the workers whether his pledges for