The "Irish question" has been at the forefront of the Brexit negotiations, even though the formal discussion about the shape of Britain’s future relationship with the European Union has not yet even begun.

With Theresa May's Withdrawal deal having suffered a crushing defeat in the Commons, it is still not clear which type of Brexit the UK will pursue – a no deal version in which the UK diverges from the EU model in search of global trading advantage, or the softer version in which the UK remains closely aligned with the EU – but the outcome of that choice will have profound impacts for the UK’s constitutional settlement.

This is true not just for Northern Ireland and its stuttering peace process, but potentially for the way that all of the UK’s other devolved constituent parts relate to each other and the political centre in Westminster.

With the help of academics, businessmen and government sources, here we look at the deep constitutional conundrum posed by Brexit for Northern Ireland, the possible solutions and the implications for the future of Britain.

So what is the problem in Northern Ireland?

The Northern Ireland peace settlement is founded on a central ambiguity: Nationalists can opt to hold Irish citizenship (and only Irish citizenship) while Unionists can remain British. In essence, Nationalists and Unionists can live together in parallel constitutional realities.