Jeremy Corbyn has never been an enthusiast for the EU. He voted against EEC membership in the 1975 referendum, the Maastricht Treaty in 1993 and the Lisbon Treaty in 2008. For decades, he stood in the left sovereignist tradition exemplified by Tony Benn. As recently as last summer, during the Labour leadership contest, he told the New Statesman that he had not “closed his mind” to EU withdrawal.

Why he eventually sided with In remains something of a mystery. Was it a principled decision? Or did the weight of EU support among the shadow cabinet, the PLP and Labour members convince him that there was no alternative? Whatever the ultimate cause, he came off the fence just four days after becoming leader last September.

Since then, Labour’s pro-Europeans have been increasingly alarmed by his reluctance to give prominence to the cause. Until today, he had not made a speech on the subject (both Miliband brothers preceded him). A recent Opinium/Observer poll found that 40 per cent of voters did not know his stance.

More will do so now. The most significant fact of Corbyn’s speech was simply his giving it. In his address at Senate House, the Labour leader frequently appeared happier attacking the Conservative government over tax avoidance and the steel crisis than he did praising the EU. But he ultimately did what was required of him. “The case I’m making is for remain - and reform - in Europe,” he said. Corbyn’s pitch was aimed at those left-wingers convinced that TTIP and eurozone austerity mean Brexit is the progressive choice.

He drew a shrewd comparison with his decision to remain a member of Labour throughout the Blair years - an institution he has since changed from within. His message to the left was that they can achieve similar feats in Brussels. "That means democratic reform to make the EU more accountable to its people. Economic reform to end to self-defeating austerity and put jobs and sustainable growth at the centre of European policy, labour market reform to strengthen and extend workers’ rights in a real social Europe. And new rights for governments and elected authorities to support public enterprise and halt the pressure to privatise services.”

To the relief of Labour’s europhiles, he also offered (by his standards) fulsome praise for the EU. “Britain needs to stay in the EU as the best framework for trade, manufacturing and cooperation in 21st century Europe. Tens of billion pounds-worth of investment and millions of jobs are linked to our relationship with the EU, the biggest market in the world.

“EU membership has guaranteed working people vital employment rights, including four weeks’ paid holiday, maternity and paternity leave, protections for agency workers and health and safety in the workplace. Being in the EU has raised Britain’s environmental standards, from beaches to air quality, and protected consumers from rip-off charges.”

There was, he concluded, “a strong socialist case” for staying in (a view that Portuguese Prime Minister Antonio Costa and former Greek finance minister Yanis Varoufakis were crucial in convincing him of). David Cameron should cheer those words. If he is to win the referendum and remain Prime Minister, he needs the left to turn out (as many as two-thirds of Tory voters oppose membership). Corbyn painted a persuasive picture of the dystopian alternative: “It wouldn’t be a Labour government negotiating a better settlement for working people with the EU. It would be a Tory government, quite possibly led by Boris Johnson and backed by Nigel Farage, that would negotiate the worst of all worlds: a free market free-for-all shorn of rights and protections.”

Corbyn is not now, nor will he ever, be an evangelist for the EU. But the Remain campaign already has a surplus of that political breed. Corbyn’s belated conversion to Brussels could prove to be an asset. Referendums are not won or lost based on the votes of the committed but on those of the waverers. Corbyn’s journey from “undecided” to “In” offers an example for others to emulate. In common with most British voters, he is a eurosceptic - but he is not a Brexiter.

Corbyn's socialist case for "remain and reform" will animate an important section of the electorate - he now needs to make it far more often.