Sorry Boris, a vote to leave means we leave

Both scenarios sound nice. The trouble is, both are implausible.

Article 50 of the Lisbon Treaty sets out a procedure for what happens if an EU member declares the intention to leave. It says that a negotiation takes place between the departing state and the rest of the EU to determine the terms of departure. Those negotiations have up to two years. If, after that time, no deal has been struck, the departing state’s EU membership automatically ceases, unless the member states vote unanimously to prolong the talks. Negotiation of better membership terms is not an option.

That’s what the treaty says. Some might respond that what matters is the politics: the other 27 states all want the UK to stay in, so they would find a way to fudge the rules to let that happen.

Well, perhaps. But let’s just think it through. One idea is that the Prime Minister might not actually trigger Article 50 in the event of a referendum vote to leave. He might say that he thinks British voters didn’t really mean it when they voted, and that he will go back to Brussels to seek a middle way.

But that is politically untenable. The options in the referendum are to remain or to leave – not to try again. Most Leave campaigners – including perhaps the majority of Conservative activists – genuinely want to leave. Failure on the part of the Prime Minister to act upon the expressed will of the people would cause uproar.

So in the event of a Leave vote, the Prime Minister must trigger Article 50.

That leaves an alternative scenario: the prime minister triggers Article 50 and formally declares the UK’s intention to leave, but then he works with his fellow EU leaders not on Brexit terms, but on improved membership terms.

"There is only one way to get the change we need, and that is to vote to go, because all EU history shows that they only really listen to a population when it says No"

Boris Johnson

One problem with this is legal: once a member state has declared its intention to leave, there is no mechanism to withdraw that declaration and prevent exit. Maybe the courts would think such a mechanism is implied – but we don’t know. If not, we would depend on perpetual extensions of the two-year post-declaration membership window – which, remember, can at any time be vetoed by a single member-state.

This gets to another political problem: the two-year limit and the requirement for unanimity to extend that limit make the UK’s negotiating position in this scenario very weak.

And if our membership lapses with no deal done, we are obliged under World Trade Organisation rules immediately to impose tariffs on our trade with EU countries. That harms us much more than it harms many of those countries. There is no guarantee in this situation that the UK could strike a better deal at all.

The same problems apply to the idea of choosing at a second referendum between the negotiated Brexit terms and continued membership. The UK has no clear legal right to reverse its initial decision. A way of fudging that might be found. But it would require unanimous support of the 27, which squeezes the UK’s power to bargain for a decent deal.

In short, a vote to leave is a vote to leave. Anyone who says otherwise is playing with fire.

Alan Renwick is deputy director of the Constitution Unit at University College London