More than two million workers on low wages are set to be relived from paying National Insurance under plans unveiled by the Conservatives.
Boris Johnson said the manifesto would include a commitment to raise the threshold at which workers start paying National Insurance contributions from £8,628 a year to £9,500. He added this figure will eventually rise to £12,500.
The Prime Minister had initially appeared to blurt out the plan ahead of the manifesto launch - expected at the weekend - during a campaign visit to an engineering company in Teesside.
He later confirmed it to reporters following him on the campaign trail, saying it would put "around £500" in people's pockets - although there is some dispute about this figure.
Mr Johnson declined to be drawn on when they intended to achieve the "ambition" of lifting it to £12,500, with the Tories saying only that it would be done "over time".
Paul Johnson, director of the Institute for Fiscal Studies, said that if the Prime Minister wanted people to gain the full benefit of £450-a-year they would have to move to the higher threshold of £12,500 at a cost of £10 billion.
Shadow chancellor John McDonnell said that while the cost to the public purse would be significant, those on low incomes would see little benefit.
He said: "Even after 10 years of cruel cuts and despite creaking public services the Tories still think the answer to the challenges of our time is a tax cut of £1.64 a week, with those on Universal Credit getting about 60p."