That decision meant there would have to be a hard border between the UK and EU in order to safeguard the integrity of the European Union's single market.
However, under the terms of the Good Friday Agreement – which brought Northern Ireland’s deadly Troubles to an end – there could not be a hard border between the north and south of Ireland.
The Northern Ireland Protocol was the attempt to square this circle and, while hardly ideal, there are times when such a political fudge is necessary.
For example, it makes no sense for Scottish MPs to be able to vote on policies that only affect England when English MPs are not allowed to vote on Scottish matters because of devolution. However, earlier this month, the system of “English Votes for English Laws” was scrapped, with the Conservatives and Labour agreeing it had been a mistake partly over concerns it was putting the future of the UK at risk.
It remains to be seen if English voters accept this, but it is clear that the Northern Ireland Protocol is causing considerable problems.
European Commission Vice-President Maros Sefcovic has said the EU’s patience with the UK’s implementation of the protocol is “wearing very, very thin” and leading supermarkets have warned that its terms could force them to switch from British to EU suppliers, with increased costs for their businesses and consumers.
Now Brexit minister Lord Frost has said the current situation “isn’t sustainable” and DUP leader Jeffrey Donaldson has called a renegotiation, saying “the barriers and distortion to trade within the UK internal market brought about by it must be swept away and not replaced”.
The protocol risks developing into a serious problem for both the Irish peace process and UK-EU relations.
What is required is a fudge that, if not exactly sweet for any of the parties, the UK, EU or Northern Ireland, is at least palatable for all. In order to avoid an escalating dispute, it may be that renegotiation is necessary. However, if they want compromises from the EU, the UK government and DUP need to recognise they may have to make some of their own.