Why David Davis quit - and how his departure may unseat the PM

I said on Thursday that David Davis had serious reservations about a Brexit plan which - although he is the Brexit Secretary - he felt he had too little role in drafting.

In particular, he was unhappy about the enduring role for the European Court of Justice in defining the meaning of the rules and regulations relating to the goods we make and buy and the food we produce and consume, which the Prime Minister is poised to propose to the rest of the EU.

I made the analogy with a Chancellor of the Exchequer whose budget had been drafted by the prime minister, not him or her - and said if that happened that Chancellor would quit.

So I said there was a strong probability Davis would resign. And so it has come to pass.

But his resignation letter, is by the standards of most such letters, astonishingly savage of a central element of the government's work - and in this case the most important policy of this or perhaps any government, how to extricate the UK from the EU.

Davis says there have been many occasions when he disagreed with her office's approach to Brexit, revealing explicitly for the first time that the biggest Brexit decisions have been taken by her and her main adviser, Olly Robbins, not by him as the ostensibly relevant minister.

So for example he disagreed with "accepting the Commission's sequencing of negotiations, through to the language on Northern Ireland in the December Joint Report".

But in the past he bit his lip and "accepted collective responsibility".

No longer.

He says "the current trend of policy and tactics" makes it "less and less likely" that May would deliver "on the mandate of the [EU] referendum and on our manifesto commitment to leave the Customs Union and the Single Market".

Ouch. Times two.

Davis continues that she has abandoned what he regarded as a firm commitment in February that the UK would have the right to diverge from all EU regulations, that she delayed the formulation of policy for life after Brexit too long, and that the backstop proposal for keeping open the border between Northern Ireland and the Republic omits "strict conditions I requested".

Here is one of his devastating attacks: "the general direction of policy will leave us in at best a weak negotiating position and possibly an inescapable one".

Which takes him to the plan agreed on Friday by the Cabinet: "In my view" he says "the inevitable consequence of the proposed policies will be to make the supposed control by Parliament illusory rather than real...The 'common rule book' policy hands control of large swathes of our economy to the EU ad is certainly not returning control of our laws in any real sense".

Worse still, he fears that the EU will now ask for an even greater erosion of UK sovereignty.

Davis felt obliged to quit because "it seems to me that the national interest requires a Secretary of State in my Department that is an enthusiastic believer in your approach and not merely a reluctant conscript".

So here is what will give the PM, at the very least, a troubled night tonight.

Davis's deep reservations with her Brexit plan are already shared by the likes of Jacob Rees-Mogg and perhaps 80 Tory MPs, especially those in the European Research Group.

And since he is the supposed authority in the Cabinet on Brexit, his articulation of their fears will swell the Brexit rebels' ranks.

So what will follow, for her and her government.

Well I am told that a new Brexit Secretary will be announced on Monday (even though a few weeks ago there were rumours that if Davis quit, his post would be abolished - and that his functions and those of his department would be assumed by the Cabinet Office and by the PM herself).

His rumoured successor is the environment secretary Michael Gove. And he won't have done his chances of landing that job any harm by the way he corralled the cabinet at Chequers on Friday to row in behind her proposals for the UK's future commercial relationship with the EU.

And perhaps Amber Rudd, recently exonerated within Whitehall for the misleading statements she made about whether there were targets for the deportation of illegal immigrants, will replace Gove.

But who to replace Davis may be the least of her worries.

Her position is threatened by his exit - partly by what he says in his letter, and partly by his relationship with her.

In her darker times, especially after her ill-fated general election, Davis has been a crucial prop to her. If he had not stood by her and urged colleagues to do the same, after she gambled away her party's majority in the 2017 general election that she didn't have to hold, chances are she would have been forced out.

So his resignation could well trigger a vote of no confidence in her by her MPs.

I reported yesterday morning that some Brexiter MPs have already written to the chairman of the 1922 Backbench Committee, Graham Brady, calling for a confidence vote.

The fact and manner of Davis's going have significantly increased the chances of the number of those letters swelling sufficiently so that she would have to - in effect - put herself up for re-election.

Her ability to lead her party and country has been seriously impaired by the departure of the minister whose work was more important than anyone else's to the prospects of her party and the prosperity of the country.

His short resignation letter may usher in the tumult of the Tory Party finding a new leader, and Britain a new PM, within a few short weeks.