Last Friday, the Belfast High Court rejected a legal challenge to the government’s claim that it can use the royal prerogative to invoke Article 50 of the Lisbon Treaty. Judgment in a similar case before the London High Court is due shortly. The Belfast court did not rule on matters which would be considered by the London High Court, but limited its assessment to the Northern Irish constitutional landscape: in particular, whether or not the 1998 Good Friday Agreement prevented Brexit being imposed on the people of Northern Ireland. A key assumption behind the Belfast judgment was that the triggering of Article 50 would not, in itself, alter the law of the United Kingdom; instead the judge found it was merely the beginning of a ‘process which ultimately will probably lead to changes in UK law’. That argument has effectively already been lost in London, since the government was forced to concede, for reasons of political expediency, that Article 50 is irreversible. Irreversibility has consequences which considerably weaken the government’s arguments on the scope of prerogative power.