The fatal mistake Theresa May made when selling her Brexit deal to MPs

The agreement to guarantee an open border in Ireland was a negotiating triumph, but the prime minister made it seem like failure

Everyone has an opinion about where Theresa May went wrong in her Brexit policy. So here is mine. I think she went wrong when she secured the breakthrough on the backstop in November – extending the temporary customs union to the whole of the UK and not just Northern Ireland.

She should have gone to the podium in Brussels on 25 November, at the end of the special summit and said: “It is game, set and match to the UK. I have secured the deal we wanted. It was a hard-fought negotiation but the EU side has conceded a UK-wide temporary customs union. That means we avoid a hard border in Ireland and we avoid a customs border in the Irish Sea. We will have the benefits of special access to the EU single market and an end to free movement of people.”

Later, when she got home, she could have been even more triumphalist. “The EU said the four freedoms were indivisible, but we divided them,” she could have told a noisy House of Commons. “They said ‘no cherry picking’ but I’ve just driven a cherry picker into the Justus Lipsius building in Brussels and taken the biggest cherry of them all. We have won control of our money, our laws and our borders while maintaining privileged, customs-free access to the single market.”

No, it doesn’t sound like her, does it? But it would have been better than what she did say at the post-summit news conference on 25 November: that she was “confident we have achieved the best deal available”. And things got worse when she got home and had to defend the deal in the Commons. Yes, the backstop is terrible, she said – and I’m exaggerating only slightly – but it will never come into effect and besides if it did it would be better than the alternative which would be to leave without an agreement.

No wonder MPs didn’t like it.

She should have sold the backstop not as a regrettable necessity but as a negotiating triumph – which it is. She should have said she hoped the backstop did come into effect because it would give us everything we want in the short term. It would focus minds on the EU side and give them an incentive to negotiate a long-term trade deal with us: to stop our firms exploiting their competitive advantage – especially those in Northern Ireland, which would for some purposes be both inside and outside the single market.

When MPs complained that Britain could not get out of the backstop without the consent of the EU, she should have said: “But you have it the wrong way round! The EU cannot get out of the backstop without our consent. We have them right where we want them. It gives us the leverage we need in the long-term trade negotiations. They will be so desperate to bring this cherry-picked arrangement to an end they will offer us good terms in the shortest possible time.”

As James Kirkup, the director of the Social Market Foundation, points out, the backstop “breaks the EU’s fundamental rules – and in the UK’s favour”. It applies only to goods, whereas 80 per cent of our economy is services. And it doesn’t apply to the economically trivial but symbolically important trade in fish.

The whole point of the backstop is to avoid a hard border between Northern Ireland and the Republic. Anyone who is opposed to the backstop needs to be asked: why do you want a hard border? (And anyone who pretends there are other ways to avoid border checks and controls needs to explain why they have failed to persuade the British and Irish governments and the EU commission.)

And the side-effect of the backstop is to keep the UK in a close economic relationship with the EU after we have left and after the transition period has ended. Who but the most extreme Brexiteers and the most extreme Remainers can disagree with that?