Iain Duncan Smith said on Friday he is resigning as Work and Pensions Secretary, complaining of pressure from the Treasury to make "indefensible" cuts to benefits.
The move comes amid mounting criticism over £4bn of planned cuts to Personal Independence Payments announced in the Budget on Wednesday.
In a statement released to the media, IDS complained that he is "unable to watch passively whilst certain policies are enacted in order to meet the fiscal self imposed restraints that I believe are ... distinctly political rather than in the national economic interest."
The minister, who has been in his post since 2010, added that he had "reluctantly" come to understand that the "latest changes to benefits to the disabled and the context in which they've been made are compromised too far."
In a direct attack on Chancellor George Osborne, IDS said that cuts to welfare were defensible as part of a deficit reduction package but not “in the way they were placed in a budget that benefits higher earning taxpayers.”
“I have for some time and rather reluctantly come to believe that the latest changes to benefits to the disabled and the context in which they've been made are, a compromise too far”
On Friday, Labour leader Jeremy Corbyn, said the resignation "reveals a Government in disarray and a chancellor who has lost the credibility to manage the economy in the interests of the majority of our people."
“The Budget has exposed George Osborne’s record of profound unfairness and economic failure," he added. "Not only must the cuts to support for disabled people be abandoned, but the Government must change economic course.
“The Chancellor has failed the British people. He should follow the honourable course taken by Iain Duncan Smith and resign.”
Earlier, the government was reportedly looking at reversing plans to cut personal independence payments (PIPs), which detractors say will leave 640,000 people up to £100 a week worse off.
George Osborne signaled the embarrassing retreat in the face of a major revolt by Tory MPs. A Government source said ministers wanted to kick the changes into the "long grass."
“The Chancellor has failed the British people. He should follow the honourable course taken by Iain Duncan Smith and resign.”
"This is going to be kicked into the long grass," the source told PA. "We need to take time and get reforms right, and that will mean looking again at these proposals."
Speaking at the European Council summit in Brussels on Friday, David Cameron argued that welfare payments to disabled people had increased by £4 billion in real terms during his time in office.
"We will always protect the most vulnerable people in our country and make sure they get the help they need,” he said. "We are going to discuss what we've put forward with the disability charities and others, as the Chancellor said today, and make sure that we get this right. It's important."
Duncan Smith's resignation letter in full:
I am incredibly proud of the welfare reforms that the Government has delivered over the last five years. Those reforms have helped to generate record rates of employment and in particular a substantial reduction in workless households. As you know, the advancement of social justice was my driving reason for becoming part of your ministerial team and I continue to be grateful to you for giving me the opportunity to serve. You have appointed good colleagues to my department who I have enjoyed working with. It has been a particular privilege to work with with excellent civil servants and the outstanding Lord Freud and other ministers including my present team, throughout all of my time at the Department of Work and Pensions.
I truly believe that we have made changes that will greatly improve the life chances of the most disadvantaged people in this country and increase their opportunities to thrive. A nation's commitment to the least advantaged should include the provision of a generous safety-net but it should also include incentive structures and practical assistance programmes to help them live independently of the state. Together, we've made enormous strides towards building a system of social security that gets the balance right between state-help and self-help.
Throughout these years, because of the perilous public finances we inherited from the last Labour administration, difficult cuts have been necessary. I have found some of these cuts easier to justify than others but aware of the economic situation and determined to be a team player I have accepted their necessity. 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.
I have for some time and rather reluctantly come to believe that the latest changes to benefits to the disabled and the context in which they've been made are, a compromise too far. While they are defensible in narrow terms, given the continuing deficit, they are not defensible in the way they were placed within a Budget that benefits higher earning taxpayers. They should have instead been part of a wider process to engage others in finding the best way to better focus resources on those most in need.
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.
It is therefore with enormous regret that I have decided to resign. You should be very proud of what this government has done on deficit reduction, corporate competitiveness, education reforms and devolution of power. I hope as the government goes forward you can look again, however, at the balance of the cuts you have insisted upon and wonder if enough has been done to ensure "we are all in this together".