Theresa May has kept the Tories happy during the Brexit negotiations – but how much longer will it last?

For Theresa May, Brexit means a double negotiation – with her own party as well as the European Union. Navigating the path to a deal with the 27 EU members is difficult enough; steering a course between pro-European Conservatives who want a soft Brexit and Brexiteers who favour a hard version makes her task nightmarish. For good measure, she needs to keep Northern Ireland’s 10 Democratic Unionist MPs on board, as her Commons majority depends on them.

Predictions that the prime minister would fail to clear various hurdles – notably, securing a phase one deal last December and a transitional agreement this month – proved wide of the mark. Low expectations heightened the sense of progress when the fences were jumped. May’s aides believe this will be crucial in winning her party’s support for a withdrawal agreement later this year. “It’s all about building momentum,” one said.

Will May definitely reach a deal with the EU? Some cabinet ministers believe privately that the collapse of the talks is more likely than it looks now there is agreement on a transitional deal lasting until December 2020. While speculation about a May “walkout” to focus EU minds has faded, some senior Tories do not rule it out. “The EU needs our money [the £40bn divorce settlement]. We may have to remind it of that,” said one minister.

The Irish border remains the most intractable problem and probable trigger of a breakdown. May could not “sell” the backstop plan – Northern Ireland remaining in the customs union and parts of the single market, with the UK border in the Irish Sea – to Tory MPs or the DUP. But the government lacks a credible alternative.

At first glance, it looks impossible for the prime minister to keep both the pro and anti-EU Tories happy about a final agreement. So far, she has managed to avoid alienating either group – partly by keeping her cards welded to her chest, to the frustration of ministers on the cabinet’s Brexit sub-committee, who grumble about her failure to give a lead.

Philip Hammond, the chancellor, leads a cabinet group wanting the closest possible links with the EU market after Brexit, and which is sceptical that trade deals with non-EU countries will bring early benefits. Boris Johnson, the foreign secretary, is the cheerleader for those wanting a clean break from the EU. The Brexit sub-committee is split down the middle, with five ministers in each camp, leaving May with the casting vote.

Can she keep them all on board when the trade talks inevitably hit a sticky patch? The most likely minister to resign in protest is Johnson. May defied the doom-mongers by brokering a compromise at an eight-hour Chequers summit last month. Brexiteer ministers were allowed to trumpet that “divergence” from EU regulations had “won the day”. It suited the prime minister to send a positive signal to backbench Brexiteers. However, May said nothing about the “managed divergence” policy in her subsequent Mansion House speech.

The Hammond-led group is relatively relaxed about the cabinet’s compromise. It suspects the EU will continue to reject such a “pick and mix” approach, forcing May to choose between moving closer to a Norway model with close EU links and an EU-Canada-style trade deal limited to goods. The pro-Europeans hope May will head down the Norway route – or at least that of Switzerland, which has good access to EU markets for goods and limited access for services. That might be too close to the EU for the Brexiteers, and, if coupled with other concessions by May on issues such as fisheries and the European Court of Justice, might see Johnson resign to lead a backbench rebellion against such a deal (with an eye on the next Tory leadership contest).

The Mansion House speech will be the model for selling the final deal to both Europhiles and Europhobes. Again, doubters believed May's address would upset one Tory faction, but she won plaudits from both by giving them something, and ensuring they could just about live with what they did not like. The pro-EU wing got May’s conciliatory language about the need for compromise, while hardline Brexiteers welcomed her rejection of a customs union. Her strategy appears to be to soften the edges of hard Brexit by as much as the EU will allow, while reassuring the hardliners that the UK is still leaving the single market and customs union.

May will need to repeat the high-wire act when it comes to “selling” the withdrawal agreement. Downing Street works on the assumption it will need the votes of all 316 Tory MPs to win Commons approval for the deal. It suspects Jeremy Corbyn will find a reason for Labour to vote against it in the hope of defeating it, and that the ensuing turmoil results in a general election.

However, many Tory MPs believe the party’s fragile truce after the Mansion House speech will not last. Crunch issues – the Irish border, a customs union, fisheries, the ECJ’s remit – cannot be put off forever. Michel Barnier, the EU’s chief negotiator, has set an October deadline for the withdrawal agreement, although UK ministers believe December might turn out to be the real one.

Which Tory faction is most likely to break with May? The Eurosceptics certainly make the most noise. Led by Jacob Rees-Mogg, the high-profile chairman of the 60-strong European Research Group (ERG), they do not like her concessions on fisheries and free movement to secure the transitional deal, but are prepared to lump it to achieve their prize of a “clean Brexit”. They will renew their threat to oust May if she makes similar concessions to get a withdrawal agreement. “She needs to remember that she needs our support,” said one ERG member. Yet their bark may be worse than their bite; they might not have the numbers to force May out as Tory leader or defeat her in the Commons.

It is easier to see the Labour opposition making common cause with the 25 pro-European Tory MPs than the Eurosceptics. That happened last December, when such a cross-party alliance defeated the government by insisting on a “meaningful vote” on the withdrawal agreement this autumn. A similar rebellion could see a majority of MPs vote for a customs union with the EU. Ministers have delayed this showdown but cannot duck it forever.

Could the pro-EU Tories join forces with Labour to reject the EU deal, or even block Brexit by calling for a referendum on it? Some in this Tory group will be tempted to do so to “put country before party”. But the majority may buckle under enormous pressure to toe the government line. May allies believe their trump card will be warning Tory rebels their actions could trigger a general election, and let in Britain’s most left-wing government ever.

Ministers believe the EU deal will be done at the very last minute. That could help May bounce Tory MPs into accepting it, as they might lack time to mobilise against her late concessions. Her rhetoric is not always matched by reality, and her red lines may look decidedly pink by the end of the negotiations. The Eurosceptics may have to face more “hard facts”, as May said at Mansion House, than they would wish. But if anyone can navigate this course without splitting the Tories, May can.