THE answer to the question whether Parliament should exert democratic control over the process of leaving the European Union ought surely to be obvious. Yet in a statement and, later, a full debate this week David Davis, the Brexit secretary, rejected repeated demands by MPs that Parliament should have a vote over the invocation of Article 50 of the EU treaty, the legal route to Brexit. Article 50 sets a two-year deadline after which Britain will cease to be a member unless the other countries of the EU unanimously agree to an extension. Mr Davis argued that the Leave vote in the June 23rd referendum gave the government sufficient authority to invoke the article, which it plans to do by the end of March.

The issue is not just political but legal, as two court cases, one in London and the other in Belfast, show. The High Court began hearing the former this week. The core argument is over whether the government is right to be relying on the royal prerogative that gives it sole authority to make (or unmake) treaties. The plaintiffs claim that the EU goes much deeper than other treaties, for instance in conferring citizenship rights. They also argue that, because Article 50 can lead automatically to Brexit, invoking it may in effect overturn the 1972 European Communities Act, breaching the principle that a parliamentary act can be reversed only by another act.

The Northern Irish case adds another argument, which is that Britain’s EU membership is entrenched in both the 1998 Good Friday peace agreement and the subsequent Northern Ireland Act. The plaintiffs in Belfast say that Britain’s membership can be overridden only by a change in the act, which requires the assent of Northern Irish voters, giving them an effective veto over Brexit (a majority in the province voted to Remain in June). The two cases are likely to be joined before the Supreme Court in London, which has already set aside two days to hear them in December.

Why is the government so resistant to letting Parliament vote on Article 50? One answer is a narrow desire to protect its royal prerogative. It is similarly loth to give the devolved administrations in Northern Ireland (or Scotland or Wales) any direct role in Brexit negotiations. Yet over the centuries the royal prerogative has been eroded, most notably through judicial review. One former senior legal adviser calls the government’s defence “surprisingly weak”, suggesting that the Supreme Court may yet rule against it.

The bigger reason for the government’s intransigence is political. Brexiteers’ deepest fear is that Remainers may yet manage to overturn the referendum result. Mr Davis argued that the 17.4m voters who backed Leave constituted the largest mandate in British history. And he said that MPs who demanded a vote were confusing accountability, which the government promises to observe through many questions and debates, with micromanagement of negotiations best carried out in secret.

The underlying problem about Brexit is that, although voters decided by 52-48% to leave the EU, they said nothing about what should replace membership. In particular, they did not vote to walk away from the EU’s single market, which is what backers of a “hard Brexit” now favour. Indeed, as Sir Keir Starmer, the new shadow secretary to Mr Davis, reminded him, the 2015 Tory manifesto that promised the referendum also pledged to “safeguard British interests in the single market”. Nick Clegg, the Liberal Democrats’ former leader, justifiably asked what constitutional principle gave the government the exclusive right to interpret what Brexit meant and to impose that on the country, rather than protecting the rightful role of Parliament.

Even if the Supreme Court does not require it, the case for parliamentary approval before invoking Article 50 remains. Last month a House of Lords report concluded that it was “constitutionally appropriate” to secure the assent of both houses of Parliament. The royal prerogative also applies to a decision to go to war; yet since the government lost a parliamentary vote on war in Syria three years ago, the convention has become that military action needs prior parliamentary authority.

As for micromanagement and secrecy, Andrew Tyrie, chairman of the Treasury select committee, correctly noted that EU talks always leak, so the government might as well publish details itself. MPs also commented widely on the irony that it should be Brexiteers, who campaigned most fiercely to take power back from Brussels to Westminster, who now seem keenest to trammel Parliament’s role in the process. Whatever the courts decide, the political battle is far from over.