Shadow chancellor John McDonnell has branded plans to cut tax credits "disgraceful" and vowed Labour would oppose them in the Commons.

MPs will vote later on the proposals, announced by George Osborne in his Budget, which have been criticised over concerns they will hit the working poor.

Labour's new leader Jeremy Corbyn claimed the policy showed the Government was intent on "punishing people for a crisis they did not cause".

The controversial shadow chancellor, whose appointment as Mr Corbyn's most senior shadow cabinet minister has caused concerns within Labour ranks, said: "The Government's cuts to tax credits are a disgraceful attack on families up and down the country.

"Labour will bring down the welfare bill, not by punishing the most vulnerable but through supporting a higher wage economy, introducing a real £10 an hour living wage, tackling high rents by addressing the housing crisis and supporting stronger trade unions to drive up pay."

He said it was an "outrage" that the cuts were being introduced without a full assessment of their impact.

In its analysis of the Budget, the Institute for Fiscal Studies think tank found that the average low-paid worker on tax credits would ''unequivocally'' lose more from benefit cuts announced than they would gain from the introduction of the National Living Wage.

Mr McDonnell said: "The IFS have said it is 'arithmetically impossible' for the Government's so-called National Living Wage to make up for these losses to ordinary working people.

"It is an outrage that the cuts are being introduced without an impact assessment and that the Social Security Advisory Committee has been denied the explanatory material and evidence it needs in order to properly scrutinise the changes."

Mr Corbyn, elected as Labour leader on an anti-austerity platform, said: "The Conservatives' cuts to tax credits will impact over three million families across Britain facing an average loss of over £1,000.

"The changes amount to introducing a work penalty into the tax credit system from a Government intent on punishing the people of this country for a crisis they did not cause."

The changes to tax credits being voted on by MPs are expected to save £4.4 billion in 2016-17.

In the Budget Mr Osborne announced plans to cut from £6,420 to £3,850 the earnings level above which tax credits are withdrawn from April 2016, as well as speeding up the rate at which the benefit is lost as pay rises.

David Cameron is set to take part in the Commons vote in an effort to protect the Government's slender majority.

The Prime Minister's official spokeswoman said the issue had been discussed at the Cabinet meeting.

She added: "Clearly this is an important part of the Government's approach to ensure that the welfare system is fair and sustainable and it is taking the action it can to make sure that work always pays.

"The Prime Minister would expect to vote this evening."