Shadow Chancellor John McDonnell has said a future Labour Government would reverse George Osborne's £4.5 billion benefit reduction after two frontbench colleagues had ducked the question.

Mr Osborne is under increasing pressure from Conservative colleagues to soften the introduction of the UK Government’s tax credit cuts.

It has been estimated that three million people across the UK, including 300,000 in Scotland, will be affected by the Chancellor's tax credit cuts and that some could lose out by around £1300 a year.

While Ministers have admitted that initially some households will lose money, they insist that, when taking into account issues like the rises in the tax-free personal allowance, an increase in the minimum wage to the National Living Wage and free childcare, they will, overall, be better off.

Ahead of a Commons debate and vote on a Labour motion calling for a halt to the tax credit cuts tomorrow, Andrew Mitchell, the former Government chief whip, argued that the "very tough" changes were necessary because the current system was "out of control".

The Tory backbencher noted: "We've got some time before it comes in so that we can tweak it if necessary but it is the right reform to make and, overall, we can make sure there are as few losers from this as possible."

He said he was sure Mr Osborne was "keeping an open mind" and could make changes in next month's Comprehensive Spending Review.

Mr Mitchell also called for "individual treatment" through the Department for Work and Pensions for those affected, saying the Government had to minimise the impact on "hard-working and low-paid" families.

There have also been suggestions Iain Duncan Smith, the Work and Pensions Secretary, also wants extra support for those who are in the run-up to Christmas also going to be informed just how much money they will lose.

Labour has claimed the changes to tax credits could put as many as 71 Tory MPs at risk of losing their seats because of the unpopularity of the policy; the 71, 21 of whom are in marginal seats, all represent constituencies where there are more families who will lose tax credits than was their majority in May.

Earlier this month at the Tory Party conference, Ruth Davidson, the Scottish Conservative leader, told a fringe meeting that she would like to see more detail on how the tax credit cuts would work in practice while David Willetts, the former Tory Minister, has warned about their impact.

Owen Smith, the Shadow Work and Pensions Secretary, stressed how many people had "deep misgivings" about the UK Government's planned tax credit cuts, including Tory MPs he had spoken to, who could back Labour in tomorrow’s vote.

When asked if he had reached out to Conservatives, he replied: "Yes, I have spoken to lots of Conservatives last week and we will be doing it again next week and some of them will vote for us."

Meantime, John McDonnell was forced to make clear Labour, if it won power, would reverse the £4.5bn tax credits cut after two of his colleagues had failed to do so.

When pressed on the issue, Seema Malhotra, the Shadow Chief Secretary to the Treasury, and Diane Abbott, the Shadow International Development Secretary, suggested the Labour frontbench had yet to take a decision on the issue.

But Mr McDonnell took to Twitter to make the announcement – another one seemingly taken without Shadow Cabinet approval – saying: “We are calling on Osborne to reverse his decision to cut tax credits. If he doesn't reverse these cuts, we're making it clear that we will."

Last week, the Shadow Chancellor performed a dramatic u-turn to oppose the Government’s fiscal charter on balancing the nation’s books, an announcement some colleagues criticised because they were not told of the shift in policy beforehand.