Boris Johnson has hinted more than 2 million low-paid workers could be lifted out of National Insurance under plans to be unveiled in the Tories’ election manifesto.

The Prime Minister appeared to blurt out the scheme to raise the threshold at which workers start paying National Insurance contributions (NICs) from £8,628 a year to £12,000 during a campaign visit to Teesside.

The disclosure came as Tories came under fire for re-branding one of their official Twitter accounts as a fact-check service during Tuesday’s TV debate between Mr Johnson and Jeremy Corbyn.

The Liberal Democrats meanwhile launched their election manifesto with a promise of a £50 billion “Remain bonus” for public services if they succeed in their aim of stopping Brexit.

Speaking during a question-and-answer session with workers at an engineering plant in Middlesbrough, Mr Johnson said the Conservatives were committed to a policy of “low tax for… the working people”.

He went on: “If you look at what we’re doing and what I’ve said in the last few days, we are going to be cutting National Insurance up to £12,000, we are going to be making sure that we cut business rates for small businesses.

“We are cutting tax for working people.”

General Election 2019 opinion polls
(PA Graphics)

Mr Johnson originally proposed raising the NIC threshold during the Conservative leadership contest last summer – although at that stage he did not put a figure on it.

However the Institute for Fiscal Studies calculated that a plan by his then leadership rival Dominic Raab to lift it to £12,500 would take 2.4 million workers out of NICs.

It put the cost to the Exchequer at between £11 billion and £17 billion depending on whether the threshold for employers was raised along with those for employees and the self-employed.

Shadow chancellor John McDonnell said that while the cost to the public purse would be significant, those on low incomes would see little benefit.

John McDonnell
Shadow chancellor John McDonnell said raising the NIC threshold would deliver limited benefits to the low paid (Steve Parsons/PA)

“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.

“Meanwhile independent experts have said this will cost up to £11 billion so everyone who relies on public services and social security will be wondering whether they will be paying the price.”

Meanwhile, Lib Dem leader Jo Swinson insisted the party’s “ambitious” manifesto plans to tackle the climate change “crisis”, invest in schools and extend free childcare showed they were more than a “one-trick pony” dedicated to stopping Brexit.

However, she told the PA news agency: “All of those things, absolutely, become much easier to do if we stop Brexit and have the benefits of remaining in the European Union.”