If a no-deal Brexit happens on 31 October, could a government that took office soon afterwards reverse it?

Boris Johnson and his top adviser Dominic Cummings think they have a cunning plan to prevent a Commons vote of no confidence scuppering Brexit. If MPs opposed to a no-deal exit force a general election, Johnson would call it for a few days after 31 October, so it is too late to stop Brexit.

It’s a clever move, but perhaps not as clever as they think. Vernon Bogdanor, a professor in government at King’s College, London, believes he has found a flaw. He argues that, if Johnson failed to win an overall majority, an incoming government could reverse the UK’s departure even though it had happened. He told Politico: “I don’t believe an election in November could frustrate a Remain or anti-no-deal position. It is true the default position in Britain and the EU is that we leave on 31 October. But the British parliament is sovereign so it can pass retrospective legislation, and it did for example with the War Crimes Act of 1991 – parliament could deem we hadn’t left and seek an extension.”

Constitutional experts are like economists: put 10 in a room, and you get 11 opinions. Bogdanor concedes that not all his fellow experts agree with him. Mark Elliott, professor of public law at Cambridge University, believes the only legally watertight way to prevent no deal is legislation requiring the PM to revoke Brexit, which MPs have baulked at. He said: “Withdrawal occurs by default operation of EU law on Halloween. Domestic legislation cannot alter that, and the EU treaties provide no legal basis for retrospectively restoring UK membership.”

However, there is always Angela Merkel’s mantra: where there’s a will, there’s a way. The EU would welcome the UK back into its family, even if technically it had already walked out. So it might decide that the UK did not have to reapply for membership.

Of course, reversing Brexit, even on the day after it happened, would cause outrage among Brexiteers. It would be the ultimate betrayal. “Riots” was one Tory MP’s prediction. Nigel Farage would surely make yet another comeback. There would probably be legal action to prevent Brexit being overturned, ending in the Supreme Court.

The prospect of retrospective legislation could ensure a very different election campaign. Jeremy Corbyn might be wary about promising to reverse Brexit, which might enhance Johnson’s appeal to Labour Leave voters in the north and midlands. But Corbyn has promised a Final Say referendum with Remain on the ballot paper. So, if the Liberal Democrats and SNP pledged to overturn Brexit and held the balance of power after the election, might a minority Labour government seek a retrospective extension of the UK’s EU membership for the six months needed to hold a referendum?

Cue more howls of outrage, of course. But the passions are equally strong on the other side of the Brexit divide. For Remainers, it would be a democratic outrage to hold a “Brexit election” in the knowledge it could not stop the UK crashing out on the eve of the contest.