British Labour Party leader Jeremy Corbyn recently laid out his party's official stance on Brexit. Notably, he announced Labour's commitment for the UK to join the EU in a customs union following the Brexit implementation period. Not only does this stance contravene important aspects of Brexit, but it means the party is turning its back on millions of its Leave-voting supporters. The democratic credibility of Britain's main opposition party has just taken a major blow, potentially damaging its future election prospects.

The Brexit referendum was remarkable in its cross-partisan nature. According to extensive exit-polling conducted by Lord Ashcroft in June 2016, about one in five Brexit voters were Labour supporters. Indeed, many felt that important political decisions affecting the UK should be made in the UK.

And yet, assuming the EU goes along with Corbyn's plans for a customs union, such an agreement would likely be conditional on the UK having limited—if any—say in the EU's rules and standards (by which Britain would have to abide to access EU markets). It would also prevent Britain from striking international deals on its own—another key Brexit demand.

Of course, Labour Brexit voters may not have as strong of opinions about trade as they do about immigration (their second most important concern, according to Lord Ashcroft). However, it isn't clear that Corbyn is interested in changing the status quo on this front either. His speech played down any expectations of regaining meaningful control over national borders.

Corbyn is often considered a voice for his party's grassroots. But, in choosing to maintain a customs union with the EU, it will be difficult for him to reconcile his party's Brexit policy with many Labour voters—notably in more economically depressed northern constituencies that voted overwhelmingly to leave.

To justify ignoring core Labour voter grievances, the calculation Corbyn and his advisers might have considered is the following: the voters this approach could anger probably won't turn to the pro-Brexit United Kingdom Independence Party (UKIP), and certainly not to the governing Conservatives in the next election, while the potential voters such a policy would attract from the Conservatives or the pro-Remain Liberal Democrats might carry Labour to a governing majority in Westminster.

Regardless of the logic motivating this decision, it seriously underestimates many Labour voters' determination to see Brexit through. The vote to leave the EU was in many ways a vote of non-confidence in a London-centric cross-party decision-making elite, which was almost unanimous in its opposition to Brexit. The signal the Labour leadership's approach to Brexit sends is that this same elite is unconcerned by the instruction it received on June 23rd, 2016: that the UK should leave the EU.

Whatever Corbyn and his frontbench believe Labour will gain by watering down Brexit, they have grossly underestimated what it could lose.

This first appeared in The Eurosceptic.