Labour, in fourth place in the polls, needs to come out for a Final Say referendum to survive

The message of the opinion polls for Jeremy Corbyn could not be clearer. The latest YouGov survey puts the Labour Party on 18 per cent, in fourth place behind the Conservatives, the Brexit Party and the Liberal Democrats.

The latest Ipsos Mori poll, meanwhile, gives Mr Corbyn the worst rating of any leader of the opposition since the series began in 1977, with 17 per cent satisfied with the way he is doing his job as leader of the opposition, and 75 per cent dissatisfied.

The Labour leader is doing something wrong. With the right of centre deeply divided between a leaderless Conservative Party and the insurgent Brexit Party, this ought to be a moment of great opportunity for a party of social justice and international cooperation.

Mr Corbyn’s ambiguous policy on Europe may not be the only cause of Labour’s woes, but it is a large part of it, not least because it has allowed the Liberal Democrats and the Green Party to capture a large part of Labour’s potential support.

The Labour leader’s refusal to get off the Brexit fence prompted public expressions of frustration from his own shadow cabinet last week. Last weekend the real cause of Mr Corbyn’s reluctance took to Andrew Marr’s sofa to explain that he is in favour of leaving the European Union.

Len McCluskey, the leader of Unite, Labour’s largest affiliate, said: “The Theresa May deal, if it encompassed a customs union, customs arrangements, which are so important in terms of investment for our nation, then that is the type of deal that I’d be comfortable to see go through.”

He said he was opposed to a no-deal Brexit but, if forced to choose between a customs union deal and staying in the EU, he would prefer to leave the EU.

Mr McCluskey’s position is at odds not just with that of most Labour members, including Labour MPs and shadow cabinet ministers, but with the views of his own members – 65 per cent of whom want to remain in the EU, according to another YouGov survey.

Mr Corbyn has indulged his ally’s stubbornness for too long. Facing both ways on Brexit is now threatening the Labour Party’s very survival as a credible electoral force.

Mr McCluskey claims, as leader of a union that campaigned for Remain in the 2016 referendum, to be a defender of democracy. But democracy is not a single event. One democratic decision can be amended by another. The decision of the 2016 referendum could not be implemented, because its advocates could not agree what it meant.

Meanwhile, another finding of the YouGov poll is relevant. It asked: “In hindsight, do you think Britain was right or wrong to vote to leave the EU?” Excluding don’t knows, 46 per cent said “right” and 56 per cent said “wrong”.

Thus Mr Corbyn needs to trump Mr McCluskey’s claim to be a democrat by advocating a fresh referendum, to give the people the final say on whether they want to leave the EU, and if so on what terms.

The Labour Party, and Mr Corbyn’s leadership, will not survive unless he backs a Final Say referendum in which the party campaigns to keep the best deal with the EU: membership.