Tory MPs concerned about George Osborne's plans to cut tax credits have been urged to join Labour in voting against the measures in the House of Commons today.

The Chancellor was challenged by Tory MPs about the £4.4 billion cuts as he came under mounting pressure to make changes to the controversial policy.

But in a sign of defiance the Treasury released figures showing that without the reforms to tax credits since Mr Osborne entered Number 11, taxpayers would face a bill in 2016-17 of almost £15bn more than is currently forecast.

In a late plea for support, Owen Smith, the Shadow Work and Pensions Secretary, and Seema Malhotra, the Shadow Treasury Chief Secretary, have written to Tory MPs, calling on them to back a Labour motion calling for the cuts planned for April to be reversed.

They claimed that around three million working families will lose an average of £1300 per year as a result of the cuts.

"This issue transcends narrow party lines, as surely none of us came in to politics to take money away from low and middle paid workers," they wrote.

"If left to go ahead, the changes will make people significantly worse off the length and breadth of the country, hitting thousands of people in every constituency. This is why representatives of all parties have raised concerns about this issue."

Tory unease about the plans was on display at a meeting of the 1922 Committee of Conservative backbenchers, where Mr Osborne was questioned about the measures.

In an effort to underline the importance of the plans as part of Mr Osborne's efforts to balance the books, the Treasury commissioned analysis on the level of public spending on tax credits that would have occurred without the Government's changes.

The figures show that before Mr Osborne's reforms since 2010, including the controversial cuts in July's Budget, spending on tax credits would have risen from £28.9bn in 2010-11 to £40bn a year in 2016-17.

It is now forecast by the Office for Budget Responsibility to be just over £25bn in 2016-17 - around £15bn lower.

Greg Hands, the Treasury Chief Secretary, said: "Labour has opposed every single saving we've made in a welfare budget they let spiral completely out of control.

"When they left office, they had allowed means-tested payments to go so far up the income scale that nine out of ten families - including MPs - were eligible.

"Treasury analysis now shows that without the reforms we have embarked on, spending on tax credits would be rising to £40bn next year. Instead it's forecast to be £25bn.

"That £15bn that we've saved - while at the same time offering working people lower taxes and higher wages, thanks to our new personal allowance of £11,000 from April and the new National Living Wage - is the equivalent to £500 extra in income tax for every taxpayer.”

He added: "Labour must now explain where the money is coming from. Their economic policy lurches further from chaos to incredibility."

Although Tory MPs were elected on a manifesto commitment to cut the welfare bill, some have concerns about the way tax credits will hit those in work.

Following the meeting of the 1922 Committee in the Palace of Westminster, Tory MP Steve Baker confirmed Mr Osborne had been asked about the issue. The High Wycombe MP said: "Lots of us don't like the tax credit changes."

He said MPs were aware they were part of a package, which also included the new National Living Wage and the increase in the tax-free personal allowance.

"But colleagues are well aware that some families will miss out," he said.

"Lots of us are very concerned about it but the vast majority of colleagues accept that these changes are a consequence of all the other pledges we made in the manifesto."

A number of Tories expect Mr Osborne to come up with plans to reduce the impact on low-paid workers.

London mayor Boris Johnson said he believed the changes were under "intensive review" while former Cabinet minister Andrew Mitchell also suggested Mr Osborne may have to "tweak" the plans.

Stevenage MP Stephen McPartland - one of only two Tories to vote against the tax credit cuts - said he believed Mr Osborne was preparing to take action to soften the blow.

Mr McPartland told the BBC: "I genuinely believe that we will be able to persuade him to tweak the policy.

"I believe he will because I believe he wants to look after those people and it will just have been a matter of maybe nobody understood the full impact."

Mr McPartland said that the prospect of a worker on £12,000 losing a 10th of his income "doesn't sit right with me", adding: "There were and still are a number of Conservative MPs who are very unhappy about the changes."

Elswehere, Tory peer Lord Tebbit, the former Cabinet Minister, said he supported tax credit reform, but noted it "depends how far and fast you want to go with this".

He added: "Clearly there should be transitioning. We are all agreed about that. Nobody is pulling the whole lot in one shot. The difficulty is to decide what is right, because there isn't surety about the numbers." 

Meantime, Paul Johnson, director of the Institute for Fiscal Studies, said: "These changes, they are certainly hitting people further down the income distribution than the changes that were made under the last parliament.

"But the Chancellor made quite a big choice in the Budget. He has decided to hit people in work rather harder than people out of work. So, he has actually made the choice relatively speaking to protect some of the poorest people on tax credits."

He added: "Wherever we end up in 2017, it's still going to be a much more generous system than we had back in 1997. The total level of spending back to about 2003 levels, not back to a world before tax credits."

In a separate development, Boris Johnson appeared to be backing away from his earlier reservations about the tax credit cuts as he emerged from a meeting of the political cabinet in Downing Street saying that reform was "essential".

The London Mayor said: "We have got to get people out of a system that is extremely cumbersome. I see people in London who are stuck in a system where they are systematically overpaid for one reason or another and then they have to pay back to the authorities.

"It's very, very distressing and we need to get out of these poverty traps. That's the reason why the reform is essential. We are in a situation where nine out of 10 families are locked into this thing.
"What we need to do is have the London Living Wage coming in and supporting those families," he added.