Well, who could have seen this coming? Just two months have passed since the agreement between the UK and the EU ended the first phase of Brexit talks and one issue already looks like it might cause the wheels to fall off the wagon – Northern Ireland. December’s agreement was always seen as a way of kicking the can down the road. The trouble is, it wasn’t kicked very far. The problem of a “hard border” between the Republic of Ireland and the UK has already come back to haunt Theresa May. The Irish border is the Gordian knot of Brexit policy, pulling together all the threads of the Prime Minister’s Brexit conundrum. The knot gets tighter the closer we get to Brexit day and it’s still not clear that the Prime Minister knows how to cut through it.

The joint agreement in December set out three options to avoid a hard border in Northern Ireland. The UK could either suggest ways to avoid a border through a comprehensive free trade agreement or by finding an as-yet undefined technological solution. If neither option proves workable, then a third “backstop” option would come into play. That would mean Northern Ireland retaining regulatory alignment with the Republic by remaining inside the Customs Union. With no proposals forthcoming from the UK Government so far, the European Commission’s draft withdrawal agreement has set out in detail how this backstop might work.

“No Prime Minister”, May told PMQs, “could accept such a deal”. That statement will come as a surprise to the EU, who saw the Prime Minister sign up to this exact proposal just a few weeks ago. To quote Ireland’s Foreign Minister, the Commission’s preparations for the backstop plan are “absolutely faithful to the political agreement in December” and should be “a surprise to no one”. Whether May is reneging on the December deal or simply trying to manage her backbench colleagues will surely be the first question asked in capital cities across the continent.

The problem is that there is no solution Theresa May could suggest that keeps everyone on board. No-one wants to see a hard border, but the UK Government doesn’t want the whole of the UK to stay in the Customs Union. The DUP, which is propping the Government up in the Commons, won’t let Northern Ireland stay in the Customs Union on its own in case that creates a border with the rest of the UK. Despite Boris Johnson’s suggestion that using technology to police the border would be as simple as London’s congestion charge zone, no realistic system has yet been outlined.

With no Commons majority and limited personal support, May doesn’t have the political capital to force any one option through but the compromises necessary to find a solution seem increasingly remote. Any suggestion that the Customs Union might remain a valid option not only risks losing the Government’s support from the DUP but also raises the prospect of a leadership challenge to Theresa May from ‘Hard Brexiteers’ on the backbenches. At the same time, the Labour party looks set to marshal its forces behind an amendment from pro-European Conservative rebels to try and keep the entire UK in the Customs Union. To say the Prime Minister is caught between a political rock and a hard place would be an understatement.

The way forward could well be the UK’s preferred solution. A fully comprehensive free trade agreement with the EU, including customs arrangements, could render the whole border debate moot. Should such a deal be achievable, however, it is still months or years away and will need to be negotiated during the transition period. Answers will be required much sooner than that. In reality, there won’t be a transition deal at all without some sort of agreement and guarantee on what might happen to the Irish border.

The decisions taken by the UK and the rest of the EU over the border now will undoubtedly shape the rest of the Brexit talks. All eyes will be on Theresa May’s “Road to Brexit” speech this Friday to see if the Government has a clear direction of travel that avoids a hard border – or if it looks like the Prime Minister has no distance left to run.