The proposed “backstop” to the Brexit withdrawal agreement is often defended on the grounds that it is necessary in order to uphold the Good Friday Agreement. But does it, rather, do the opposite?

That’s the view of Lord Trimble, who has now said that he plans to bring legal proceedings to this end. He’s one of the architects of the Agreement, although that doesn’t make his view definitive: nationalist parties in Northern Ireland and the Irish government, who were also part of the Good Friday process, have the opposite interpretation. But let’s look at the case he makes.

Trimble’s basic claim is that the “backstop” would violate the principle of “consent” in the Good Friday Agreement, by changing the basics of the Northern Ireland political settlement without the approval of the Northern Ireland population. It’s one thing to make this as a political argument – this change ought to be subject to Northern Ireland’s approval – and another thing to make the legal claim that it must be.

The biggest problem with the legal claim is that the Supreme Court has already indirectly ruled against it. In the well-known Gina Miller case, which was decided along with arguments involving Northern Ireland, the court was asked to rule that Brexit violated the Good Friday Agreement principle of consent.

The court looked at the Northern Ireland Act, which includes the rule that “Northern Ireland in its entirety remains part of the United Kingdom and shall not cease to be so without the consent of a majority of the people of Northern Ireland”. In the judges’ view:

This important provision, which arose out of the Belfast Agreement, gave the people of Northern Ireland the right to determine whether to remain part of the United Kingdom or to become part of a united Ireland. It neither regulated any other change in the constitutional status of Northern Ireland nor required the consent of a majority of the people of Northern Ireland to the withdrawal of the United Kingdom from the European Union.

Although the case concerned Brexit, the judgment more broadly suggests that the consent principle in the Good Friday (Belfast) Agreement, as set out in the Northern Ireland Act, was only about Northern Ireland joining Ireland as such. It did not apply to Brexit, and more generally did not apply to “any other change in the constitutional status of Northern Ireland”. This would seem to rule out the argument that the principle of consent applies to the backstop. And this is hardly judicial activism: the plain words of the Northern Ireland Act, quoted above, refer only to Irish unity as regards the principle of consent.

Ironically, Lord Trimble might have a stronger case based on EU law. There’s an argument that in the absence of a definite end date, or possibility of unilateral termination, the backstop goes beyond what the EU can regulate in a withdrawal agreement, which is not meant to cover a departing Member States’ future relationship. There’s a recent briefing by a Magic Circle law firm to this end, which also argues that the backstop might infringe the principle of “peace” in EU law. The counter-argument to that is that the Irish government – and until recently the UK government – believed that the backstop was necessary to secure peace.

In order to raise this point though, a national court in the UK would have to ask the EU’s court to rule on the issue, and it’s not clear if the EU court would say it has jurisdiction to rule in a case from a national court on a treaty that isn’t in force yet. Moreover, there’s precious little time for the EU court process (or even a national court process) to deal with such arguments. It might make sense for an opponent of Brexit to hope that a last-minute court case will somehow delay the process, but many Brexit supporters are opposed to a delay.

So while you can never be certain what a court might say on an issue until the judgment comes out, Lord Trimble faces some obvious hurdles in the way of making his argument.

Steve Peers is a professor of EU, human rights and world trade law at the University of Essex. He tweets @stevepeers