Theresa May is presently juggling a multitude of political problems. First, there is the controversy surrounding her decision to join air strikes against the Syrian regime without parliamentary approval. Second, the ongoing Windrush Scandal has engulfed the government in recent weeks, which some attribute to the potentially discriminatory ‘hostile environment’ policies she championed during her tenure as Home Secretary. Third, after a substantial defeat in the House of Lords, the government has once again been forced to reassure Brexiteers that the United Kingdom will leave the single market and customs union.

Indeed, after an alliance of Labour, Liberal Democrat, crossbench and 24 Conservative peers combined to support an amendment by Lord Kerr to require government to ‘report back’ to Parliament on its progress in arranging an EU customs union, it now seems that parliamentary support for the UK remaining within the customs union is the biggest threat to the government’s management of the final act in the Brexit negotiations.

May has always been clear in that the UK must leave the customs union and not join any similar arrangement post-Brexit, though this debacle demonstrates that the government has been unable to articulate a convincing argument for why this is the case. It is true that if the UK were to remain within the customs union, the ability to negotiate bilateral trade deals outside the union would be severely curtailed. Likewise, after leaving the EU, the UK would be obliged to follow the terms without having a say in future EU free-trade negotiations. Equally, if the UK were to agree a partial-customs union based on the Turkey model, it could exclude agriculture and services and automatically open the UK market without providing reciprocal rights.

However, there are several challenges to this position that the government has hitherto been unable to solve. Remaining within a customs union would avert the economic drawbacks to Brexit through ensuring frictionless trade with a proximate trading bloc and providing access to the EU’s 56 free trade agreements. A customs union would also help deliver a solution to the Irish border conundrum, the chief obstacle to the government’s hard Brexit vision. Lastly, the government’s proposed solutions are unfeasible, with the EU27 making it clear that a either a ‘customs partnership’ or maximum facilitation ‘MaxFac’ arrangement would be insufficient to avoid a hard border.

It is for this reason that government faces a series of parliamentary defeats over the issue and some of May’s senior advisers have reportedly conceded privately that a U-turn could be inevitable if the government appeared likely to lose a vote in the Commons. However, there are three reasons why this position would be untenable.

First, hard-line Brexiteers headed by the Eurosceptic European Research Group of Conservative MPs have privately and publically warned the Prime Minister that they would back a vote of no confidence if the government changed course.

Second, government officials have predicted that while some Cabinet Brexiteers such as Michael Gove and David Davis would accept a U-turn, Boris Johnson and Liam Fox would both resign, again likely triggering a leadership challenge and government collapse.

Lastly, the DUP, which demonstrated its power to halt Brexit negotiations in December 2017 when Arlene Foster refused to support any deal that would see Northern Ireland leave the EU on diverse terms to the UK, has recently stated that if Northern Ireland were forced to stay in the single market or customs union (its Brexit ‘red lines’), the party would vote against the government.

Whilst delivering Brexit and keeping her party together and building a cross-party coalition in the process has been a benchmark she has repeatedly set herself, notably in her January 2017 Lancaster House speech, the non-binding Commons vote today and the very real prospect of future parliamentary defeats signals a widening split between parliament and the government over May’s Brexit approach, leaving Theresa May’s position looking increasingly precarious.