The clear message from a large poll of trade union members is that most want a fresh referendum and the chance to keep the UK in the EU

Thirty years ago this week, Jacques Delors came to Bournemouth to urge Britain’s trade unions to change their stance on Europe. The president of the European commission told TUC delegates that the EU was good for workers’ jobs, workers’ rights and workers’ living standards. It was a decisive moment in the union movement’s relationship with Brussels.

This week could be equally decisive for the TUC – perhaps even more so – given the precarious balance of forces at Westminster. And the clear message from YouGov’s poll of more than 2,700 members of the TUC’s three biggest unions is that most trade union members think Brexit is bad for jobs; they want a fresh public vote and the chance to keep the UK in the EU.

Can we be sure that YouGov’s figures are right? Do the people it polled accurately reflect the views of all the members of the three big unions? I recall the same questions being asked when YouGov first showed Jeremy Corbyn well ahead in the race for the Labour leadership three years ago. Nonsense, said the critics. YouGov’s respondents, they claimed, were hopelessly biased towards leftwing activists.

When it came to it, Corbyn won by almost precisely the majority reported in the final poll. And the methods YouGov used in the latest union survey are essentially the same as it used in Labour’s leadership election three years ago.

Moreover, the new surveys find that union members’ views are much the same, whether they are men or women, young or old, north or south, white-collar or blue-collar. If the data were adjusted to a different demographic profile, the overall percentages would move by no more than one or two points.

We can therefore be confident that most members of Unite, Unison and GMB (which have a combined membership of more than 3 million people) really do want the UK to stay in the EU, and a chance to vote in a fresh referendum once the negotiations are complete.

Their reasons are clear. It’s not that trade union members are indulging in gesture politics or ideological breast-beating. They are worried about the impact of Brexit on jobs, taxes, living standards and the NHS. They fear a Brexit Britain would find it harder to sell products and services abroad.

Their attitudes to immigration are especially significant. In the 2016 referendum, one of the arguments for Brexit was that immigrant workers were undercutting the pay of low-paid British workers. Brexit, so the argument ran, would allow Britain to stop this. As a result, there would be more, and better-paid, jobs for British workers.

Many Unite, Unison and GMB members earn below-average wages. They might be expected to support that part of the Brexit agenda. They don’t. Overwhelming majorities, ranging from 74% to 85%, want EU citizens either to have complete freedom of movement to come to the UK, or the freedom to settle here if they have a job or university place lined up.

The politics of Brexit are, of course, complex. The three big unions are major, and influential, supporters of the Labour party. They cannot, and arguably should not, ignore the wider political context in which they have to decide their stance on Brexit and calls for a people’s vote.

Equally, they face the possible consequences of a Brexit that could affect their members more than anything that any British government has done since Margaret Thatcher’s heyday more than 30 years ago. What YouGov’s research shows is that most members of the big three unions know this. This week’s debates at the TUC in Manchester will show whether the leaders of those unions are marching in step with their members.

Peter Kellner is a former president of YouGov