“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,” Iain Duncan Smith, former UK Welfare Secretary

Iain Duncan Smith has resigned from the British government after the annual budget included a £1.3bn attack on disability benefits. Here’s what it means in five bullet points:

Austerity has hit the buffers. Its aim, according to Conservative economic theory, is to kickstart growth. But it hasn’t so they need more austerity. At some point the austerity vs humanity problem was going to trigger the conscience of a Tory minister and this is it. The background is the vicious Tory infighting over Europe. Given the Cameron faction is using the whole bag of dirty tricks against the Johnson/Duncan Smith faction over Brexit, IDS has clearly had enough. There will now be a strong challenge to Cameron after 23 June, whether Britain votes to stay or leave. Osborne’s budget is unravelling. The education secretary Nicky Morgan last night suggested billions of pounds worth of cuts were “suggestions”. She had to cut short a TV interview today. My long engagement with Westminster leads me to see this as circumstatial evidence of a wider civil war inside the Conservative government over the scale of pointless austerity Osborne is imposing to reach his — clearly unreachable and stupid — fiscal targets. This is Jeremy Corbyn’s victory. In one speech he’s blown apart the Tory front bench, made likely two substantial revolts, destroyed the cabinet and made the Tories look like incompetent fools. And the weekend is young: it’s probably not over. IDS’ letter to Cameron draws the logic clearly. It’s a disaster for Blairites. They’d prepared their cabbage patches of opposition to Labour’s own new fiscal rule, and spent weeks revving up to diss Corbyn over his expected mishandling of the Budget. Instead Labour is ahead in one poll, tied in another, and its radical left leadership looks not just vindicated politically, but — and this matters in the Commons — tactically: Corbyn and McDonnell executed a near perfect hit on the government by announcing their own fiscal rule; denouncing the benefit cuts; and now splitting the cabinet.

It is no longer “put up or shut up” time for the Progress wing of Labour. Just the latter.

As for the Cameron government: its disarray tonight is of a different order to, for example, the Heseltine resignation of 1986. That happened while Thatcherism was in its ascendant. This happens while Cameronism — whatever that actually is — has descended into ideological and governmental disarray.

And — the bottom line — because of IDS’ position as a Brexiteer: the whole episode, on top of today’s disgusting EU deal over Turkey — gives momentum to Brexit. Watch my twitter feed @paulmasonnews as it all unfolds.