CHANCELLOR Philip Hammond is standing by his tax grab on Britain’s self-employed, despite a growing rebellion among his own party.

According to the Treasury’s own analysis, the increase in Class 4 NICs will mean 1.6 million people, roughly a quarter of all Britain’s self-employed, will now pay an average of £240 a year more. Other analysis suggested the true figure will closer to 2.5 million people.

The move directly contradicted a commitment in the Conservative Party manifesto at the last general election not to increase National Insurance contributions (NICs).

Tory whip and Wales minister Guto Bebb said the Chancellor should apologise for breaking the party’s promise to voters.

Speaking to the Welsh language BBC Radio Cymru, Bebb said: “I believe we should apologise. I will apologise to every voter in Wales that read the Conservative manifesto in the 2015 election.”

Other senior Tories, including former party leader Iain Duncan Smith, called on the Government to rethink the proposal.

At one point yesterday afternoon, it looked as if a U-turn was on the cards, but the Chancellor is increasingly bullish.

However, Labour say any change to National Insurance needed to pass in a dedicated bill which would require a vote in the House of Commons. Theresa May only has a majority of 17 and could be vulnerable to a rebellion “Certainly the Labour party will oppose this,” Shadow Chancellor John McDonnell told BBC One’s Breakfast show. “I think other parties will as well. We may be able to persuade enough Conservative MPs to ask the Chancellor now to think again.”

The SNP’s economy spokesman, Stewart Hosie, called it a “tax on ambition”.

“In recent weeks the Tories have been arguing for tax breaks for the wealthiest in Scotland, all the while storing up a tax hike on the self-employed and planning to break their manifesto commitments.

He added: “Many self-employed in Scotland do not earn large amounts of money and have had to become self-employed in response to changing labour market conditions and new ways of working and do not benefit from paid sick leave, holiday pay or other employee benefits.”

The row overshadowed grim analysis of the Treasury’s sums by independent thinktank the IFS.

In a stark warning to Britain, they said the workers were on course for 15 years of lost earnings growth.

Hammond’s budget, and Brexit would result in “a third successive parliament of austerity”.

“On current forecasts average earnings will be no higher in 2022 than they were in 2007. Fifteen years without a pay rise. I’m rather lost for superlatives. This is completely unprecedented,” IFS director Paul Johnson said at the thinktank’s s traditional post-budget day briefing.

He added: “All of the productivity – and with it earnings growth – we would normally expect has been lost forever. This remains the big story of the last decade – a decade without growth, a decade without precedent in the UK in modern times,” he said.

The IFS said the UK was on course to borrow £20 billion in 2020 – £30bn more than intended a year ago.

“That leaves a lot of work to do in the next parliament to get to the planned budget balance. It looks like being, I’m afraid, a third parliament of austerity,” said Johnson.

Earlier in the day the Resolution Foundation said that the UK is in the midst of the worst decade for pay growth since the early 19th century, when Britain was at war with France.

Torsten Bell, the director of the Resolution Foundation, said: “The big picture from [Wednesday’s] budget is that the big squeezes on both the public and family finances have been prolonged well into the 2020s.

“While the Office for Budget Responsibility at least delivered some good news on borrowing, the family finances picture has actually deteriorated since the autumn statement. Britain is set for a return to falling real pay later this year, with this decade now set to be the worst for pay growth since the Napoleonic Wars.”

Bell said it would be the poorest who were hurt more: “The combination of weak pay growth and over £12bn of benefit cuts means that for the poorest third of households this parliament is actually set to be worse than the years following the financial crisis.”

According to the Foundation’s calculations, a single person working full time on the minimum wage – earning £13,150 – will be £380 worse off by 2020. A couple with two children and combined earnings of £29,020 will be £360 a year worse off by 2020.