Boris Johnson following David Davis out the door gives those Tory Brexiteers who want to storm the Downing Street barricades all the spur to action they need.

Mr Johnson took a while to make his mind up, arguably many months. It’s not the most dignified resignation perhaps, waiting for Mr Davis to take the lead and then pondering the pluses and minuses of a move now.

No. 10 had calculated that it could face these two resignations and might be able to survive them. But it can’t be sure. It can be reasonably sure now that an attempt on the PM’s leadership will be made. Forty-eight MPs need to send in letters demanding a vote of no confidence. That now looks very plausible.

WIll others now rally to the PM? What, for instance, will the new Brexit Secretary, Dom Raab do? Mr Davis has been heard to boast of Mr Raab as a political clone and protégé. Did he recommend him to the PM as his successor simply to increase Mr Raab’s chances of getting a chance at an even bigger job if and when the PM is toppled?

Mr Davis has denied to friends that he tried to persuade Mr Johnson to resign when he called him last night before announcing his own resignation. The suggestion is that the phone chat was “desultory” but not incitement. But Mr Davis will have known that with others alongside him his departure would carry a better chance of shifting policy. He spoke to other senior Leave ministers in the same round of phone calls.

His phone call with Mrs May late last night started with some polite small-talk but Mrs May seems to have known the game was up and though she twice told him of her disappointment that he was going she didn’t try to stop him.

And now she has the another big Brexit beast gone from the Cabinet. Theresa May’s aides always said that Mrs May brought Boris Johnson and others in to key Brexit-related jobs to keep them from making trouble for her on the backbenches. They were there to give Brexit cover to a Remain-voting PM. Both those needs remain but they don’t.

Can Theresa May survive a challenge if it comes now? There were allies who thought that was possible and said that they relished the opportunity. They may be about to get the chance to test their strength.