This photo marks the moment it all went wrong for Theresa May

Even if Brexit goes badly, I don't think the Prime Minister would accept that she should have done anything differently

Theresa May signs the official letter to European Council President Donald Tusk invoking Article 50 (Reuters)

We had that Ed Balls in the back of our seminar at King’s College, London, the other day, and he said the worst feeling in politics was that of suspecting that you were making a mistake, as you were making it. He was talking about the changes to Labour’s leadership election rules, but now there are many of The 48 Per Cent who feel that the nation is in the middle of making one of the worst mistakes in its history, and that Theresa May will come to regret that photo of her signing the Article 50 letter in the Cabinet room, with a Union flag behind her.

This is most unlikely. For one thing, even if she is a Remainer trapped in a Brexit body, which is an over-simplification, she would say that she had no choice. Even Alastair Campbell, the most forward of the anti-Brexit echelons, said this week: “Article 50 is the process that follows legitimately from the referendum.” If there was a mistake, it was not in starting the Article 50 process – which she has delayed for a remarkably long time with hardly a squeak of protest from the Leavers – it was either in the referendum vote itself, which was not May’s responsibility, or in her interpretation of what it meant, which certainly was. However, anyone who suggests that the referendum was a vote to leave the EU but to keep the free movement of people between Britain and the EU has a lot of work still to do.

Even if Brexit goes badly, I don’t think May would accept that she should have done it differently. I am told that David Cameron tells anyone who will listen that he does not regret promising and holding the referendum. He feels he had no choice, and I think that is true both tactically and strategically. Tactically, he could not have held his party together, fended off Ukip and won the 2015 election without promising a referendum. But strategically, the referendum was coming because the people wanted it and the political system was slowly bending to that democratic imperative – the Conservative Party was becoming an anti-EU party.

She could have said, I suppose, I don’t want to be the prime minister who takes us out of the EU because I don’t believe in it. But that is, as I say, an over-simplification. She, like Boris Johnson, was torn. She thought there were many good reasons for leaving the EU, democracy, sovereignty and control of immigration being three of them. And she thought there were strong arguments for staying, the economy and the possible break-up of the UK being two. On balance, she thought we should stay, but 52 per cent of the people weighed the balance differently, and she is not betraying her principles to recalibrate her scales from 48-52 one way to 52-48 the other.

If Brexit does go awry, she may regret the way she handled the negotiations over the next 18 months. If a deal cannot be reached that satisfies her, the EU 27 (operating by qualified majority voting) and the European Parliament, that might be a problem. If a temporary trade deal cannot be reached that satisfies all of the above, that would be a severe economic shock to us and a bad economic outcome for them. And the trade deal, which is separate from the Article 50 procedure but which will be negotiated in parallel, has to be agreed unanimously by the EU 27 countries. That gives each country a veto, including in some cases parts of countries such as Wallonia, the French-speaking half of Belgium which recently blocked and forced changes to the trade deal between Canada and the EU.

So we have to hope that Philip Hammond, the Chancellor, was right this morning when he said, “There will be a deal,” and that he was talking about the trade deal, which is much harder than the Article 50 deal on terms of exit (reciprocal rights of EU and UK citizens and settling the check-out bill).

If there is a deal, however, I doubt that people will look back on the moment that May signed the letter as a mistake. There will be an economic cost to Brexit even then, but it will not be a sudden shock. Over a ten-year period, Britain will be less well-off than it otherwise would have been, but people will still be better off than they are now. And for The 52 Per Cent, confirmation bias is a powerful thing.