THE biggest surprise of Iain Duncan Smith’s resignation is that it did not happen sooner.

IDS has been the public face of some of the Government’s most toxic welfare ‘reforms’, but he has repeatedly clashed with the Chancellor since becoming Work and Pensions Secretary in 2010.

For Duncan Smith, overhauling the social security system had a moral dimension: he believed the welfare state had created dependency and wanted the system to encourage work.

George Osborne saw reform in terms of cold numbers and deficit reduction. With pensioners protected from austerity, cutting the welfare budget for working age adults was the easy and obvious target for the Treasury.

IDS has defended and promoted the Government's welfare agenda – think of his staunch support for the bedroom tax – but the cuts would have been deeper had he not stood up to Number 11.

Sources close to both Tory heavyweights have, through leaks and indiscreet comments to biographers, shed light on an on-off battle that has continued for six years.

Matthew d’Ancona’s In It Together – an account of the Tory-Liberal Democrat coalition – quotes a conversation between Osborne and a friend on IDS: “You see Iain giving presentations and realise he’s just not clever enough.” Osborne denied the claim.

However, the author also claimed the Chancellor was frustrated with IDS. “He opposes every cut,” the book quoted Osborne complaining.

D’Ancona also argued, in his own words, that IDS believed Osborne had been “captured by the Treasury and its institutional arrogance”.

However, there is a strong suspicion in Tory circles that IDS was looking for a way out and the row over Personal Independence Payments provided the opportunity.

IDS, a gut euro-sceptic, is determined to campaign vigorously for Brexit until the referendum in June.

Leaving the European Union is one of the reasons IDS is in front-line politics, but being in a Government committed to the EU restricts him.

By resigning, he can become a stone in the Chancellor’s shoe as well as promoting an issue he has been obsessed about since the days of John Major.

The self-styled Quiet Man has found his voice and is directing it at the residents of Downing Street.