Theresa May is no longer in control of events. The Brexit withdrawal agreement she is defending in Parliament at present has, with reason, been savaged by all sides and seems likely to be rejected in the vote planned for next week. Already, thoughts are turning to Plan B.

The best course is evident: a second referendum, one that would let Britain choose more wisely now that the costs of Brexit are better understood. But the path back to sanity is by no means straightforward, and one obstacle in particular needs to be removed.

The European Union should make clear that the decision to quit the EU can be reversed.

There was never a good way for May to deliver the Brexit that the country voted for in 2016. Still, the deal she concluded with the EU was worse than almost anybody — Leavers and Remainers alike — had expected.

It threatens to leave Britain with most of the obligations of EU membership and few of its privileges; it separates Northern Ireland from the rest of the UK in a kind of economic annexation; and it fails to provide an exit clause, a future Article 50, by which Britain might end these arrangements.

Mervyn King, former Governor of the Bank of England, is not exaggerating when he calls these terms a betrayal of Britain.

The EU’s leaders charged their negotiators to drive a hard bargain, which they did, and then some.

So hard, in fact, that Britain’s voters seem unlikely to swallow it. Therein lies the opportunity to salvage something from this wreckage.

A “people’s vote” to reverse the Brexit decision looks increasingly possible — but it requires three main things.

First, the British Government must either advocate it or let it happen.

May and her perpetually divided Cabinet may be unable to reverse themselves on this, as she has repeatedly said there will be no second vote.

Thankfully, this may not matter. In one of several humiliating defeats the prime minister suffered this week, the House of Commons asserted the right to vote on alternative proposals if MPs vote down her plan.

Second, the EU needs to remove several procedural obstacles.

In particular, its leaders need to say that if Britain revokes its Article 50 notification of withdrawal — as it would have to, to give time for a second referendum — the EU will not object.

Up to now, the European Commission and Council have argued before the European Court of Justice that the revocation would require the unanimous assent of the other EU members.

This week, in a boost to Remainers, the advocate general of the ECJ said this was not so, though the guidance is not binding on the court. Europe’s leaders should affirm that they’ll let Britain revoke its notice to quit.

Third, the EU’s leaders should resist the understandable temptation to drive another hard bargain, this time over getting Brexit reversed.

They should say that Europe wants the UK to remain, and that it can do so following a second referendum on the same terms as before — with no obligation to join the single currency, with its other opt-outs preserved, and with no change to the prevailing financial terms.

There are those in Europe who would like to see the back of Britain, perpetual objector to the union’s grander ambitions. Even its friends might wish to see those annoying concessions eliminated as the price of readmission.

However, this would risk turning voters back in favour of Leave, snatching defeat from the jaws of victory.

Forgive and forget would be the wiser course. After all, Europe will have made its point. Thinking of quitting the EU? Britain tried, and see what happened?

If Brexit goes ahead, especially in its most damaging no-deal form, Britain and the EU both lose.

If it can be reversed, which is looking more possible by the day, both gain. A smart and timely intervention by Europe’s leaders could make the difference.