Philip Hammond is facing growing pressure to back down on a £2 billion National Insurance hike for the self-employed, as former work and pensions secretary Iain Duncan Smith added his voice to calls for a rethink of the move.

In a round of post-Budget broadcast interviews, the Chancellor insisted that the decision - along with a cut in tax-free dividend allowances which will also hit the self-employed - was "fair and appropriate".

But with at least 10 Conservative backbenchers voicing misgivings about the Budget measure, Labour made clear that it hopes the threat of rebellion will force the Chancellor into a U-turn.

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Mr Duncan Smith told Sky News he hoped the Chancellor would "reflect" on the decision in the period before the autumn Budget.

"I would like to see this kept under review ... I would like to see the ball kept in play," he said.

"This doesn't land until next year, so there is plenty of scope to look at how this actually affects them and to listen to business representatives."

The change, which will cost 2.5 million self-employed people an average £240 a year, was savaged by normally Tory-supporting newspapers as a clear breach of a 2015 manifesto promise not to increase National Insurance contributions (NICs) for five years.

Describing the announcement as a "shocker", shadow chancellor John McDonnell told BBC1's Breakfast: "I'm hoping that we will be able to persuade the Chancellor to back off from this.

"Certainly the Labour Party will oppose this. I think other parties will as well. We may be able to persuade enough Conservative MPs to ask the Chancellor now to think again."

And several Tories broke ranks to demand a rethink before the changes come into effect in April 2018.

Berwick MP Anne-Marie Trevelyan told the BBC Radio 4 Today programme that the Chancellor was "going in the wrong direction".

She added: "We need to halt this particular decision now. I think we need to put this on hold so we can have a proper review and think in a holistic way."

And Stevenage MP Stephen McPartland said the move was "taxing those families who have taken on the risk of setting up their own small business, many of which employ apprentices and are the backbone of our economy."

Cleethorpes MP Martin Vickers told the Grimsby Telegraph he was not "overjoyed" by the NIC hike, while backbenchers Tom Tugendhat, Jacob Rees-Mogg and Nigel Mills, along with former minister Andrew Murrison, intervened in Wednesday's debate in the Commons to voice concern.

Meanwhile, Bromley & Chislehurst MP Bob Neill told the Press Association: "It needs to be kept under review ... We need to keep an eye on it because if it does start squeezing the genuine White Van Man, that is something we need to look at."

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And former small business minister Anna Soubry suggested the dividend allowance cut from £5,000 to £2,000 could be Mr Hammond's "first U-turn".

"This will not be popular and many will argue it's unfair," she said.

With Prime Minister Theresa May's working Commons majority of 17 making the Budget measures vulnerable to any backbench rebellion, former shadow chancellor Chris Leslie said Labour MPs were already in talks about a cross-party amendment to block the NICs hike.

Mr Leslie said he had been informed by the House of Commons Library that the change would require primary legislation.

Mr Hammond said he was "prepared to listen to backbenchers" on the issue, but insisted the move was designed "to make the National Insurance system a little bit fairer".

The Chancellor said the gap in benefits received by self-employed workers and employees which has historically justified their different treatment by the National Insurance system "no longer exists", except in relation to parental rights which he has pledged to review.

And he denied the increase from 9% to 11% in Class 4 NICs for the self-employed amounted to a breach of the manifesto pledge, insisting the issue was "behind us" because legislation passed after the election had made clear only contributions by employees were protected.

Mr Hammond told LBC radio: "This is a fair measure, it is a modest measure.

"Half of the money raised will come from people in the top 20% of income earners.

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"Nobody earning less than £16,250 will pay any more National Insurance contributions.

"Sixty percent of self-employed people will see a reduction in their National Insurance contributions.

"This is a fair and appropriate measure."