Boris Johnson visits Newton Aycliffe and Teesside on the General Election campaign trail

Mr Johnson has visited washing machine manufacturer Ebac in Newton Aycliffe while election campaigning, ahead of the General Election.

The Prime Minister met workers and tried his hand at moulding parts at the Ebac plant, which is owned by outspoken Brexit supporter John Elliott.

Mr Johnson posed through drums and placed "Made in Britain" stickers on new machines as he toured the factory - the UK's only washing machine manufacturer.

Mr Elliott has previously challenged both Helen Goodman and Phil Wilson, the Labour MPs in Bishop Auckland, where he lives, and Sedgefield, where the factory is based, over their stance on leaving the EU.

• Boris Johnson visits The Northern Echo in Darlington

Mr Johnson moved on to Teesside, where he toured Wilton Engineering, which sits in the shadow of Middlesbrough's Transporter Bridge.

The Prime Minister appeared to blurt out the plan to raise the threshold at which workers start paying NI from £8,628 a year to £12,000, during the campaign visit.

Speaking during a question-and-answer session with staff at an engineering plant in Middlesbrough, he said the Conservatives were committed to a policy of "low tax for... 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."

Mr Johnson originally raised the idea of upping the threshold for NI contributions during the Conservative leadership contest last summer - although at that stage he did not put a figure on it.

The Institute for Fiscal Studies (IFS) calculated that for each £1,000 that it was raised, 600,000 workers would be lifted out of NI altogether.

If just employee and self-employed thresholds were lifted the cost to the Exchequer would be £3 billion a year rising to £4.5 billion if the employer threshold was raised as well.

To raise the threshold to £12,500 - as Mr Johnson's then leadership rival Dominic Raab was proposing - was put at £11 billion, rising to £17 billion if the employer threshold was included.

The IFS said that 2.4 million workers would be taken out of NI.

Boris Johnson at Wilton Engineering Picture: JULIA BREEN

Crowd of engineers at Wilton Engineering waiting to speak to Boris Johnson Picture: JULIA BREEN

A group of protestors gathered outside the gates shouting 'Tories out' and 'no more cuts'.

A dozen people, who said they were local residents, stood outside the factory holding a sign saying "stop lying Boris" as they chanted.

They stood around a hundred yards from where the PM is due to speak to workers, and voiced concerns about education among other issues.

One of the protesters shouted: "We want funds for our schools, we want bus services for our children to get home and to college."

Another man shouted: "Stop your lying."