The resignation of Iain Duncan Smith ‘proves beyond doubt how wedded the Tories are to austerity’, the SNP has said.

The Conservative MP resigned from his role as UK Work and Pensions Minister on Friday. Stephen Crabb was announced as his replacement on Saturday morning.

The Scottish National Party called on Prime Minister David Cameron to ‘immediately and completely’ scrap cuts to disability benefit announced in Wednesday’s budget.

Dr Eilidh Whiteford MP, SNP spokesperson on Social Justice and Welfare, said: "Iain Duncan Smith's resignation proves beyond doubt how wedded the Tories are to austerity as a political choice, and how far they are willing to go to balance the budget on the backs of disabled and disadvantaged people.

"If the latest cuts to disability support are too much for Iain Duncan Smith, the architect of the bedroom tax, it shows the Tories need to rethink their whole approach to welfare reform, and abandon their ideological commitment to austerity cuts.

"Disabled people must not be made to pay the price of politically driven austerity or Conservative civil war over Europe.

"David Cameron must take control of his divided government and confirm that these reckless and poorly thought-out cuts - the latest in a long line of cuts to disability support - will be scrapped completely."

Scottish Conservative leader Ruth Davidson said she 'wholeheartedly' welcomed Stephen Crabb's appointment as Work and Pensions secretary.

She said: "He knows from first hand experience that, for millions of people in this country, life can be a daily struggle. He is the right man for this job. We need a government that works for everyone and gives them a fair crack of the whip and I know Stephen is committed to that.

"It's now vital that he is given the time and opportunity to build on the successes this Government has had in reforming welfare. Unlike Labour and the SNP, the Conservatives have sought to tackle the many failings of our welfare state, getting thousands of long-term unemployed people back into work. We must ensure the route back into a job is always worth the effort. And we need to reassure people who can't work that they will be given the support and dignity they deserve.

"I hope Stephen's appointment marks the start of a fresh focus – right across government– on building a better welfare state. We must not let the huge long-term benefits of welfare reform be sullied by short-term cuts. With Labour failing to offer any kind of credible opposition, people are looking for a competent party of government, and the Conservatives must be the party that speaks for all the UK, and for all working people."

In a letter to the Prime Minister, Mr Duncan Smith said he felt policies were being carried out for political reasons rather than in the national interest.

He said: "I am unable to watch passively whilst certain policies are enacted in order to meet the fiscal self-imposed restraints that I believe are more and more perceived as distinctly political rather than in the national economic interest.

"Too often my team and I have been pressured in the immediate run up to a budget or fiscal event to deliver yet more reductions to the working age benefit bill.

"There has been too much emphasis on money saving exercises and not enough awareness from the Treasury, in particular, that the government's vision of a new welfare-to-work system could not be repeatedly salami-sliced."

Calling the latest changes to benefits "a compromise too far", he said: "You are aware that I believe the cuts would have been even fairer to younger families and people of working age if we had been willing to reduce some of the benefits given to better-off pensioners but I have attempted to work within the constraints that you and the Chancellor set."