Iain Dale presents ‘Drive’ on LBC Radio.

“What an utter copper-bottomed shit”. Those were the words that greeted me when I took a call from a Tory MP a couple of minutes after Iain Duncan Smith’s resignation was announced on Friday night. My caller fulminated about the self-indulgence of a man who dared to ruin the Chancellor’s less than carefully crafted budget. He even dared to echo the Chancellor’s apparently long held view that the former Welfare Secretary wasn’t quite the full shilling in the intelligence department.

Those of us who inhabit the Westminster village are always prone to ask in these circumstances: ‘what did he mean by that?’ We look for hidden or disguised motives. We assume ulterior motives where there often are none. More often than not, ministerial resignations are not long-planned and are more likely to be the political equivalent of a hissy fit. Alternatively, they are the culmination of a long grinding down by the Prime Minister or the Treasury.

Let’s not beat about the bush. Iain Duncan Smith knew this was his last hurrah in mainstream politics. He was not in it to gain preferment to something greater. Having led the Conservative Party, there wasn’t really anywhere else for him to go. He had no ambition to be Chancellor or Foreign Secretary. This put him in a strong position. Both the Prime Minister and Chancellor both knew that there was only so much they could do to push him down a policy road he didn’t wish to travel. There was no love lost between him and the Chancellor. They had fought some bitter battles over the years. Unfortunately, George Osborne has always underestimated Iain Duncan Smith and this time there were catastrophic consequences.

On the left, IDS has taken on an iconic status as a hate figure. The myth has grown up that his mission in life was to hurt poor people, by reforming the welfare system so the poor got less. Oh, how they misjudged him. Yes, he got religion on welfare reform. He was passionate about it because he had identified how the welfare state had let the poor down over a number of decades. He couldn’t understand why the Tories had ceded this ground to Labour, whose urban councils had presided over decades of decline in council estates all over the country. He saw the client state they had built up. He believed Labour ensured that poverty ruled in certain areas in order to preserve their vote. Glasgow was a prime example. He knew that if he could reform the system, and drag people out of semi-permanent poverty, things could be very different. This underlay his whole approach. Yes, he thought the welfare bill could be reduced. In many ways it had to be if the deficit was to be reduced. But to him reforming the system of welfare was just as important as cutting costs. That’s where he parted company with HM Treasury and the Chancellor. All they cared about – and have ever cared about – is raw numbers. Hang the long term, cut in the short term.

The Treasury has always tried to assert its dominance all over Whitehall. It happened under Nigel Lawson and Gordon Brown. But under this Chancellor, the Treasury’s power is all-pervading. Nothing is done without the Treasury’s say-so. David Cameron has effectively ceded control of domestic policy to the Chancellor, and he’s imposing himself in every department. One cabinet minister told me they are not allowed to say or do anything without running it by Osborne’s special advisors. In the end something had to give, and many other cabinet ministers will be silently cheering IDS for exposing what is going on, not just at the DWP but all over government.

People question the timing of this resignation, but is there ever a right time? IDS could have gone at any time over the last two years, but chose to hang on in there in the hope that he might prevail. Nadine Dorries has tweeted her displeasure at IDS’s efforts last week to persuade her to support his welfare cuts. He said it would be a personal betrayal if she didn’t. And why resign on the day when it seemed the Treasury appeared to weaken on the PIP payment proposals? Perhaps it was one battle too far.

Could part of the reason have been Europe? Many Tory MPs suspect that the referendum had more than a little to do with the timing. Up to a point, possibly. There’s little doubt that Duncan Smith has been unhappy at some of the comments the Prime Minister has made about the motivations of the ‘outers’ and his dismissive conduct of the arguments IDS has put forward. Resigning from the government means that IDS will be in a position to take a much more high profile leadership role in the Leave campaign and this may well have been a minor factor in his decision.

In the end, however, the language in his resignation letter said it all. It took on a Geoffrey Howe-esque tone and seemed to invite others to search their own consciences too. There is one crucial difference. This wasn’t planned. Priti Patel, IDS’s deputy at the DWP, was at an Asian Business Awards Dinner on Friday night. At around 9.05 her phone buzzed. It was IDS. She rushed out of the hall at London’s Park Plaza hotel to take the call, looking somewhat ashen-faced. He hadn’t pre-warned his LEAVE campaign colleague what he was about to do, nor anyone else it seems, probably because he didn’t want anyone to have the chance to persuade him not to do it.

IDS has never been massively popular among his fellow parliamentary colleagues. They regard him as somewhat aloof and above it all. Among the Tory grassroots it is somewhat different. They still feel guilty about his overthrow back in 2003 and he’s incredibly popular at Conservative constituency association events. They are the same people who are highly suspicious of George Osborne, who they’ve never quite warmed to.

One consequence of this resignation is confirmation that the Downing Street fear factor is on the wane. I was at Michael Ashcroft’s 70th birthday party last Saturday where I encountered a Minister who had disobeyed Downing Street’s instruction not to attend. “You’ll be on a little list,” I joshed. “I really couldn’t give a toss,” came the reply. And it was heartfelt.

Yet more evidence that power is gradually ebbing away from Cameron and Osborne. The question is, who is the power ebbing to?