The key that would unlock a referendum on the terms of Brexit is the Labour Party. If the official opposition were to support giving the people the final say, then it would become likely that it would happen. Without that support, a new referendum is something that could come about only if the Brexit talks collapse in crisis.

Hence the advice from the People’s Vote, the leading group that is campaigning with The Independent for another referendum, to its supporters to refrain from attacking or undermining Jeremy Corbyn.

This is wise advice. It ought to be obvious – sadly in this polarised, social media age it appears not to be – that the way to persuade people is to work with them, to engage respectfully and to build the widest possible alliances.

In this case, when the stakes are so high and Mr Corbyn has already kept his options open, it would be profoundly against the national interest for the debate to be contaminated by the sectarian grievances that are currently tearing Labour apart.

Mr Corbyn’s history as a Eurosceptic is not irrelevant, but he has never been dogmatically opposed to the European Union itself. His criticisms of it in the past have been for its weak accountability, its tendency as he saw it to favour capital over labour, and its rules against state aid for industry.

Those are all pertinent arguments, and lent credibility to his stance in the 2016 referendum of being, on balance, in favour of the UK’s continuing membership. He has been criticised for an ambiguity born of trying to appeal to Leavers and Remainers alike, which he did surprisingly successfully in the 2017 general election. But if there is to be another referendum, the pro-EU side may need as much of that coalition as it can get, and Mr Corbyn would be important in helping to deliver it.

It was notable, too, that in recent interviews the Labour leader refused rather inelegantly to say whether he thought the UK would be better off out of the EU. The most plausible reading of such reluctance is that he believes the opposite, but does not want to offend Labour supporters who voted Leave. Supporters of a Final Say should cheer his implied view and applaud his attempt to keep the Labour coalition together.

The debate in the Labour Party is building up to a crucial phase at the annual conference in Liverpool next month. Opinion polls suggest that 78 per cent of party members support a new referendum. Many constituency parties have asked the conference to debate the question, and most trade unions have not ruled it out.

Most important, perhaps, Momentum, which began as Mr Corbyn’s leadership campaign, will meet this weekend to decide whether to put the question to its members, after 4,000 of them signed a petition calling for a conference vote.