This month the current UK parliamentary session has become the longest since the English Civil War (1642-51). It is a fitting distinction: the political civil war convulsing Britain is now in full swing, and is likely to get worse.

The clarity over Theresa May’s departure and the collapse of the cross party talks do not provide any clearing of the air: these developments simply confirm Brexit’s deepening distempers.

Those who have clung to the centre are now heading to the extremes, pulled by forces the leadership of both parties can no longer control.

The Tories are haemorrhaging voters to Nigel Farage because they haven’t delivered Brexit; Labour is losing out to the Liberal Democrats, Greens and Change UK because of Jeremy Corbyn’s ambiguity on a second referendum.

Both the leave and remain camps are convinced the momentum is on their side.

Purist thinking, the pre-requisite to any sleepwalk into catastrophe, is now dangerously fashionable.

"Britain is a desperately divided country right now," Simon Coveney, the Tánaiste, told reporters in Brussels this week.

"The issues are still as complex as they always have been, and to find a way forward everybody has to be willing to compromise.The problem is that some on the UK side don’t seem to be willing to do that. They want a purist view, and they won’t accept any compromise to that view."

EU officials talk of capitals being in "suspended disbelief" at what is happening.

At least, according to a pessimistic reading, there is grim clarity to what happens next.

The Tories will be humiliated in the European Parliament elections.

Theresa May will make one final abortive attempt to get the Withdrawal Agreement through House of Commons by way of the Withdrawal Agreement Implementation Bill (WAB). 

A leadership contest will follow in which the two front runners will place themselves before the party grassroots in a run-off.

Boris Johnson announced on Thursday that he would contest the Tory party leadership election

Given that the Tory grassroots have swung truculently behind no deal, a rhetorical arms race will catapult a Boris Johnson or a Dominic Raab into Number 10 on the promise of a showdown with Brussels.

The EU will reject any attempt to re-open the Withdrawal Agreement, the new prime minister will threaten no deal, but parliament will thwart that it.

"It’s impossible to know how this will pan out," says one London source close to the Brexit process. "It will be extremely messy and very drawn out, by the time you’ve cleared the MP process [in the leadership race] and then the party members’ process. It will be a time of chaos for the country."

Time is in short supply, and a bruising autumn will come around very quickly.

"We assume the leadership contest will start early and that the Tories will want it completed by the summer recess," observes a senior EU official.

"We will see a very nervous start of the fall with the party conference season in the UK, and then there are just two weeks left before the European Council in mid-October."

That is the last chance the UK will be able to seek a further extension to Article 50, which expires on October 31.

The dynamic might change, but the outcomes look stubbornly familiar.

"The Europeans will be confronted by a more Brexiteer prime minister, or at least one with stronger credentials than May in terms of delivering Brexit," says the official.

"The Tory party will be in an existential crisis after the European elections, so the build up in October will be pretty rough.The new prime minister may not therefore come with a request for an extension, but to tell us this is the last attempt to make a deal, or tinker with the deal, and if not there is no deal.

"All for the sake of the survival of the Tory Party."

The official stresses: "Whoever the new leader is, the political margins to ask for another extension will be very limited."

This will push all sides out of their comfort zone.In dealing with extension requests up to how, EU leaders have had the luxury of tactical options.

Any granting of an extension was exclusively designed to nudge the House of Commons towards accepting the Withdrawal Agreement.

Back the treaty or you’ll have to contest the European Election.Back the treaty or you face a longer extension and the risk that Brexit might not happen at all.

This was the political calculation in Brussels (and Downing Street).It hasn’t worked.

"Now we may end up with a new leaders who says, it’s the Malthouse Compromise [already ridiculed and rejected by the EU], or it’s nothing, and then it’s no deal by the end of the year," says one EU diplomat.

"That will probably inject a lot more electricity into the European Council than we had before."

However, a Tory leadership contest does not change the arithmetic in the House of Commons.

Parliament remains opposed to Britain leaving the EU without a deal.

Business, which will in October be facing into a highly uncertain Christmas retail season, is also adamantly opposed to no deal.

Faced with such a deadlock in the House of Commons a desperate last-gasp demand for another extension cannot be ruled out. 

But this would place the EU in a very difficult position. 

Emmanuel Macon, the French President, would feel vindicated in his resistance to a long extension back in April and in his belief that Brexit would really start to contaminate the European body politic.

Ireland, and the other countries most exposed to no deal, like the Dutch, Belgians and Danes, will argue strongly for an extension.

Donald Tusk, the European Council President, is understood to be in favour of an extension to the end of June to give the UK time to sort the mess out.

But after that the risk curve for EU policy-making starts to rise.

And in any case, Tusk’s term in office comes to an end in the autumn.

So, another extension in October poses risks for both sides.

"I can’t see a new British prime minister coming and saying, give us another three months, without a serious explanation as to what it’s for – a general election, a second referendum or whatever," says the EU diplomat.

"But at the same time, as a more hard Brexit pm he or she will have to seriously justify domestically why they are asking for it.Even Boris would lose his appeal at that point.

"The European Council is not in the mood to kick the UK out. We’ve demonstrated it twice. If someone else wants another demonstration it would probably come."

So either a no deal stand-off in the House of Commons or a last minute extension request could trigger a collapse in the government followed by a general election.

The Brexit Party, emboldened by their anticipated success in the European elections, would scoop up more Tory votes.

The same dynamic would hurt Labour: the threat of more of their remain-minded supporters switching to the Liberal Democrats would mean the party membership demanding a confirmatory vote be part of their election manifesto.

For that reason, the Tories in particular would be desperate to avoid an election.

Donors have dried up and party activists are demoralised.

Neither party would command an overall majority, and the House would again be divided along tribal rather than party lines: Hard Brexit or Second Referendum. 

Either way the agony would simply drag on. A no deal exit would, according to all respected economic forecasts, deal a shocking blow to the British economy.

According to the EU’s assessment, London would simply have to come back to the EU desperately seeking a new trading relationship, and the EU would place the priorities of the Irish border, a financial settlement, and citizen’s rights right back on the table as a pre-condition for talks getting under way.

Could any of this be avoided?

The optimists – a hardy bunch to be sure – are looking at the mechanics of Theresa May’s final roll of the dice: bringing the Withdrawal Agreement Implementation Bill (WAB) before the House of Commons on the week beginning June 3.

This is the legislation to enshrine the divorce treaty into UK domestic law,

Under Section 13 of the EU Withdrawal Act, Downing Street has been obliged to give MPs a "meaningful vote" on the final treaty.

The first two meaningful votes combined the Withdrawal Agreement and the non-binding Political Declaration on the Future Relationship as one package to be voted on by MPs.

On 15 January it was defeated by 230 votes and on 12 March it was defeated by 149 votes.

Speaker of the House of Commons John Bercow

For the third meaningful vote, Downing Street was forced – under pressure from the Speaker John Bercow – to decouple the Withdrawal Agreement from the Political Declaration.

Despite the Strasbourg agreement between Theresa May and Jean-Claude Juncker, the Commission President, providing legally binding clarification on the backstop, the Withdrawal Agreement suffered a third defeat, this time by the narrower margin of68 votes.

What Downing Street is proposing now is, rather than have a fourth straightforward attempt to get the treaty over the line, the government would instead present the actual legislation that would enshrine the Withdrawal Agreement in domestic law.

What difference would this make?

There are some merits.It would spell out in graphic form what is involved in legislating to make the treaty happen, and it could force MPs to fully understand what the treaty involves.

It also offers a degree of flexibility by way of numerous amendments that could reconcile the conflicting demands revealed in the cross party talks: how to make Brexit softer, how to approach the idea of a permanent customs union, how to ensure workers rights and environmental protections are enshrined for the future, and how to make sure the House of Commons has a greater role in the trade negotiations, if and when they get underway.

Soft Brexiteers in the Tory Party and Leave-supporting Labour MPs may be so scarred by the European election results that they might see a last opportunity to deliver an orderly Brexit as one worth taking.

Crucially, too, it might give the new Tory leader the political cover to avoid a no deal and further gridlock.

"There are different incentives," says Jill Rutter, of the Institute for Government.

"Everyone is gaming out multiple scenarios about what it means, where you’ll be in a Tory leadership campaign, where you’ll be in an election campaign".

In other words, a new Tory leader – even a hard Brexiteer – might recoil from having the very first months of their premiership being tarnished by an immediate door slamming in Brussels, a crisis in the House of Commons, and the last minute humiliation of another extension.

If MPs grudgingly swallowed the WAB, the new Tory leader could plausibly say, well, Theresa May was an appalling negotiator, but at least we have delivered Brexit and now I can hit the ground running and be a much tougher negotiator in the trade negotiations.

MPs on both sides could load up other amendments that have been floating around Westminster in the past months in the hope they could command support across the House. 

These would appeal to both sides of the debate: Labour could table an amendment calling on the future relationship negotiations to deliver a permanent customs union, or even that the Withdrawal Agreement could be ratified on condition that it was put to a second referendum.

Other amendments could include a promise that the House of Commons would be more closely involved in those trade negotiations, and that workers’ rights and the environment would be protected.

On the Brexit side, Dominic Raab has suggested that the payment of the financial settlement should be conditional on progress in the trade negotiations.

An amendment by Hugo Swire MP in December suggested the House of Commons be asked to give its approval for the backstop to take effect.

Some MPs might be tempted to go further and seek to strip the backstop out altogether, or to resort to the so-called Brady amendment and have it replaced by "alternative arrangements".

Obviously these latter suggestions would be out of the question for the EU.

It’s clear the WAB approach would quickly unravel if there was any suggestion of amendments unpicking aspects of the Withdrawal Agreement.

By going down the WAB route Downing Street could feasibly get around the statutory requirement of having a meaningful vote by simply have a clause that says one is not necessary.

But anything in the bill which seeks to alter any elements of the Withdrawal Agreement will convince the UK is simply not capable of ratifying the treaty according to international law.

Clearly the backdrop is not conducive to Labour throwing Theresa May a last minute lifeline. 

The two main parties spent six weeks trying to square the circle, and while senior Tories talked about a constructive working relationship largely thwarted by uncompromising backbenchers on both sides, the well has been poisoned, and the gap between those in Labour who want a second referendum and a permanent customs union, and those in the Conservative Party who want a hard Brexit is just too wide.

Boris Johnson, or whoever succeeds Theresa May, will face existential choices once enthroned at the head of a stricken Tory Party.

The reality, more daunting than the comfort of the backbenches, may well paralyse the newcomer.

The English Civil War, ending as it did with the beheading of Charles I and, before long, the restoration of the monarchy, offers few instructive parallels.

Oliver Cromwell, when asked about his personal fortunes, opined: "No one rises so high as he who knows not whither he is going."

Perhaps that is the best advice currently on offer.