British politics is polarised on nearly every axis, so it is strange how little conflict there is between Labour and the Conservatives on the biggest issue: the terms of departure from the EU. Jeremy Corbyn’s challenges to Theresa May over Brexit negotiations at prime minister’s questions last week felt remarkable because he so rarely opens battle on that front. Labour has not obstructed Tory legislation enabling the very hardest of Brexits. The frontbench say they would pursue a different model, putting “jobs first”. But whips have instructed Labour MPs to sit on their hands as the Tory agenda is enacted.

Mr Corbyn’s views in the area are vague, except to insist that for democracy’s sake, the referendum verdict must be honoured. That is a sensible starting point for the leader of a national party, especially one that represents many areas that voted leave. But ending EU membership leaves a spectrum of options, notably in the question of the single market and customs union. The Tories are dedicated to rupture from both; Labour equivocates.

There is a view on the left that the single market, with its rules governing industrial subsidy, prohibits the kind of nationalisations envisaged in Labour’s last election manifesto. This is a misconception (as the prevalence of state-run enterprises across the continent shows). As for the customs union, the objection voiced by the shadow international trade secretary, Barry Gardiner, is the mirror image of the one raised by Liam Fox, his government counterpart. Both men want out of the customs union because member countries cannot sign external trade deals.

Dr Fox likes the idea of bilateral deals because they will allow greater deregulation, less protection for workers and fewer restraints on multinational business. Labour’s priority in any post-EU deal would be bolstering those rules and restraints. Meanwhile, UK insistence on leaving the customs union is the direct cause of Mrs May’s current difficulties in Brexit negotiations in relation to the island of Ireland. Were she to soften on that front, the prospect of a restored border and the impasse go away. Challenged on this point on Sunday, Dr Fox insisted the border matter be deferred for a later phase of talks, although the fundamental problem will not change. Mr Gardiner also swerved the question, saying Labour had ruled nothing out, while adding that the opposition was excused from taking a position because it was not a participant in the negotiations.

For the Tories there is at least ideological consistency in flight from the single market and customs union. Dr Fox’s wing of the party has long desired a bonfire of European social protection. Labour’s history and values should tilt the party in the opposite direction. In an interconnected world, the best strategy to avoid a race to the bottom in workers’ conditions is international cooperation, using the combined muscle of many governments to police global capital. This was a historic benefit of the European project for the left – and is why most large trade unions campaigned to remain. Labour’s ambitions of upholding social protections are best met in political and economic alignment with the EU. As Mr Corbyn demonstrated on Wednesday, Labour has already travelled some way on Brexit. Now it is time for the opposition to bring a defence of the single market and customs union out from the shadows of the Tory hard Brexit agenda.