THERESA May is “fully committed” to pressing ahead with her controversial plan to hike National Insurance Contributions for the self-employed, No 10 has insisted, after she was accused by Labour of being in disarray over the policy.

It has emerged the legislation to bring in the measure has been pushed back to the autumn.

The Prime Minister’s declaration came as video footage appeared to show David Cameron hitting out at his Downing Street successor, saying: “Breaking a manifesto promise is stupidity.”

With flak from both Opposition and Tory benches over the NICs rise - set to cost 2.5 million Britons an average £240 a year - Mrs May has sought to dodge the issue of a broken manifesto pledge not to raise the tax that pays for pensions by focusing on her argument that creating a better level playing field on NICs between the employed and self-employed was fairer.

However, with a report due in the summer from Matthew Taylor, a former policy chief to Tony Blair, on the possibility of extending more rights to the self-employed, Mrs May has made clear new legislation to bring in the 2018 changes will now not take place until later in the year.

This has led John McDonnell, the Shadow Chancellor, to accuse the Tory Government of being in “disarray” and of making a “partial U-turn”.

A number of Conservatives have voiced concerns about the £2 billion hit on the self-employed -announced by Philip Hammond, the Chancellor, in his first Budget - with even a minister, Guto Bebb, saying the Government should apologise for breaking an election manifesto commitment not to increase NICs.

With dozens of Tories thought to be angry about the Budget measure, Labour made clear that it hoped to force a full U-turn in the face of the threat to the PM’s 17-seat Commons majority.

Mrs May has acknowledged the Budget had meant "difficult decisions" but insisted it was vital to close the gap between the amount of tax paid by the self-employed and those in "traditional" employment.

Asked how committed the PM was to the NICs hike, given the strength of opposition, her spokesman replied: “She is fully committed.” Asked if there would be any watering down of the proposal, he added: “No.”

One of the Conservative critics of Mr Hammond's proposal, Tonbridge & Malling MP Tom Tugendhat welcomed Mrs May's readiness to listen to backbench concerns, saying: “The policy has got to reflect the individual risk that entrepreneurs and self-employed people bear...It has got to reflect the reality of modern employment."

But Mr McDonnell sensed a U-turn in the offing. "The fact the Prime Minister won't fully support her own Chancellor's Budget measure and has been forced by Labour to row back on it just 24 hours after he delivered his speech in Parliament shows the level of disarray that exists at the top of government."

Meantime, it emerged that Mr Cameron appears not to have been impressed by the Government’s plan to hike NICs. The former PM famously declared on the 2015 General Election campaign trail that a forthcoming Tory Government would not raise NICs.

Earlier this week during a memorial service in London for those killed in the Iraq and Afghanistan wars, Mr Cameron was caught on camera, appearing to say: "Breaking a manifesto promise is stupidity."

Asked about his comments, the PM’s spokesman said: “Lip-reading is not in my job description.”