Budget 2016 Iain Duncan-Smith
Former secretary of State for Work and Pensions, Iain Duncan Smith, resigned today citing unfair cuts Carl Court/ Getty Images

Work and Pensions Secretary Iain Duncan Smith has resigned, saying that he was under increasing pressure to make cuts whilse the latest Budget "benefits higher earning taxpayers". A letter of resignation was issued by Duncan Smith on 18 March, a crucial time as George Osborne's budget comes under increased scrutiny.

The 61-year-old's resignation is a huge blow to the Conservatives, who have angered many over £4bn of planned cuts to Personal Independence Payments (PIP), which are expected to affect 640,000 people. The government has said that they will "reconsider" the cuts after widespread concern.

Osborne has insisted the "most vulnerable" in the UK will still be protected and the overall disability budget was increasing, but he will still contact colleagues and disability charities to "make sure we get this absolutely right".

Jeremy Corbyn tells George Osborne to follow Iain Duncan Smith and quit over disability cutsIBTimes UK

But Duncan Smith, who had been in the job almost six years, clearly does not want to see the results of these meetings, saying the cuts were "not defensible" within a Budget that "benefits higher earning taxpayers".

The changes, outlined in Wednesday's budget, will adjust the critical formula that the government uses to work out the daily living component of PIP. The new allowance is set to replace Disability Living Allowance (DLA) in January 2017.

The letter reads: "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.

"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," he said in his resignation letter.

"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."

The former Conservative Party leader and Leader of the Opposition from 2001-2003 had locked horns behind closed doors with the treasury over some of the cuts. The Edinburgh-born Duncan Smith also criticised 'pessimistic' David Cameron and George Osborne over the EU referendum.

Osborne claims the new budget would allow for an extra £1bn on disability despite other figures revealing would save £4.4bn by 2020-21.