MONDAY’S day of great drama leaves Britain’s weakest government for years leading the country towards its most momentous decision for decades.

Theresa May’s premiership is now hanging by the slenderest thread but her desperate weakness has long been her greatest strength. She has been able to withstand embarrassing buffetings which would have killed off lesser mortals because no one expects any more of this minority Prime Minister. Even last night, as the latest calamity unfolded around her, she let it be known, in a Thatcheresque tone, that she would fight on and on and her careful compromise would not be altered.

Indeed, one has to admire her obduracy. Despite her cabinet colleagues’ lack of support, she has battled determinedly on, just as she battled on through her buttock-clenching nightmare of a conference speech.

More than a year ago, former Chancellor George Osborne chortled that she was so weak that she was a dead woman walking – but none of her opponents, on either side of the House, has yet found a way of burying her.

One commentator has described her as “the Japanese knotweed of Prime Ministers because she digs in so hard”.

But yesterday as she addressed the Commons, her cabinet unravelled and there was open civil war on the benches behind her, while the number of letters from her MPs calling for her to go neared the crucial 48 mark. And there was one big change.

The reason Mrs May has been able to cling to her thread is because there has been no better alternative – that’s why Jacob Rees-Mogg, a parody of a politician, has been touted as a favourite to succeed her.

But yesterday, David Davis’ resignation smoked out Boris Johnson. With Mr Davis out, Mr Johnson couldn’t remain in, chained by collective responsibility to a policy he doesn’t believe in – after all, this man of memorable phrases described Mrs May’s Chequers agreement as “polishing a turd”.

The question now is what sort of campaign Mr Johnson is going to run. Is his resignation just an attempt to force Mrs May to reshape her soft policy and harden her approach to Brexit? Or is he going to run for the top job himself?

Mr Johnson is still popular with Eurosceptics and his charisma means the public still warms to him. He was recently secretly recorded calling for more Trumplike guts in Britain’s negotiating position, so sitting quietly on the backbenches will not look very presidential.

His resignation letter was hard hitting, speaking of how under Mrs May’s soft Brexit, his dream of freedom was dying and of how Britain was becoming a colony of the EU. But it wasn’t quite a demand for Mrs May to go.

That’s because, for all his bluster, Mr Johnson is a prudent fellow. He clearly spent the weekend working out if this really was the right moment for his resignation, and he must know that even if does force the moment to its crisis and topple Mrs May, he would still be hamstrung by the same thin Parliamentary arithmetic as Mrs May.

The only way that arithmetic can change is by calling a general election. The Tories have in recent months been slightly ahead of Labour in the polls, but the British people have become very unpredictable in recent years. As Mrs May proved last June, it would be a foolhardy politician who gambled even a 20-point lead on the outcome of the ballot box.

At the moment, Labour – which has its own deep divisions over Brexit with its leader as compromised over his party’s direction as Mrs May is over hers – is a sideshow to the great Conservative psycho-drama. What, for example, is Michael Gove’s tactic? He was one of the architects of the Brexit campaign but over the weekend surprisingly came out strongly in favour of Mrs May’s soft compromise. Perhaps he knows that the Conservatives don’t take kindly to treachery. Having knifed Mr Johnson’s first leadership bid, he doesn’t want to get a reputation as a serial back-stabber and so is now professing loyalty to his leader.

For the time being at least.

And, in promoting Dominic Raab to the role of Brexit secretary, did Mrs May yesterday unwittingly anoint her own successor?

The irony is that David Cameron called the in/out referendum to heal the decades-old Euro-fault in his party, but instead it has blown those deep divisions wide open like a volcano, spewing out anger like red hot lava. It is inevitable that Mrs May will be swept away by that lava flow, but when is impossible to predict.

And all the time, the deadline for the most important decision in Britain’s post-war history is rushing towards us with all the speed of a World Cup semi-final.

Not even Gareth Southgate would be able to make a silk waistcoat out of this pig’s ear.