When a party is in opposition, especially in the first year after a bad defeat, not much of what it says matters to the rest of the country. It is not to disparage Jeremy Corbyn to say that this is the situation that Labour currently faces. It would be the same under any other leader. This is a period of rebuilding for Labour. And these are early days.

Britain’s EU referendum is a different kind of political event. Here, Labour has relevance in an unusual way for so early in the political cycle. In the referendum all votes count equally and the result shapes the country’s future. It is therefore a moment when what Labour says matters. It is also a chance for Labour to give a clear signal about the kind of country it wants Britain to be.

The referendum campaign is a contest in which Labour votes have special importance. Much of the Tory party is against Europe whatever the deal that David Cameron brings back from Brussels. So are most Ukip voters. Labour therefore has a pivotal responsibility to give positive leadership, particularly in England. Make the right case for Europe, and make it well, and Labour can be the engine of victory for the remain campaign. Get it wrong, or imply that the result does not really matter, and Labour may find itself inadvertently helping the Brexit cause.

Mr Corbyn is by instinct more Eurosceptic than his party. In this respect he is out of step not just with most Labour MPs, but also with most trade unions and most Labour voters. Since becoming leader he has, however, recognised that this is not the issue on which to pick a fight with the party. This has been to his credit and to Labour’s benefit. Now is not the time to change that approach. However much the Conservative party deserves censure for its Europe policy, and the EU merits criticism for its economic failings, from this week on, Europe is not a Tory cause but a national one.

The Labour leader is due in Brussels himself this week. He has scheduled meetings with fellow European socialists, all of whom are keen supporters of the EU, and all of whom want Britain to remain. This has long been Labour’s own position too, rightly so. Labour’s pro-European case is based on big strategic progressive pillars – including shared security against climate change and crime, cross-border solidarity among peoples, and making best use of Europe’s collective strength in the world. Above all it is in the interests of the people Labour speaks for. All these things would be threatened by leaving the EU, in spite of its genuine faults.

It is important Mr Corbyn takes this opportunity to restate the Labour case plainly, clearly and without distraction today. Nobody should expect him to campaign ardently for something he manifestly does not believe. But he must ensure that he does nothing in the campaign to give succour to the anti-European cause. The country’s future, as an outward facing, open and pluralist nation depends upon the consistency of such an approach.

Just as no one should expect Mr Corbyn to suddenly become an ardent pro-European – however much that would be welcome – so also it is important that other Labour leaders step up to make the case the party believes in. The party was right to choose Alan Johnson to be the voice of Labour’s campaign. The former home secretary is trusted, recognised and a good communicator. If he hasn’t already cleared his diary through to June, he should do so immediately. This campaign not only needs Labour leadership; it needs a fully energised Mr Johnson, along with many others, to supply it.

When the referendum comes, every vote will matter. Labour must not appear or be indifferent or uninterested. This must be Labour’s campaign on its own terms as well as David Cameron’s, Nicola Sturgeon’s and the other parties on theirs. Labour should be straining every sinew to win the vote to remain. This is a moment of national importance. It’s time for all shoulders to be put to the wheel, and none of them is more important than Labour’s.