EU referendum: Shadows of Tony Benn and Bob Crow loom large over Jeremy Corbyn

Jeremy Corbyn's surprise Labour leadership election victory was thanks to a surge in anti-establishment, left-wing support last summer. With the New Labour project abandoned and Ed Miliband's top flight political career in ruins, the party's membership decided to go radical. But seven months later, their advocate risked trashing his 'right on' credentials by throwing his support behind the campaign to keep the UK in the EU.

Corbyn had never kept his Euroscepticism secret and his late socialist allies, trade union chief Bob Crown and Labour veteran Tony Benn, railed against Brussels as a 'capitalist club'. The trouble for the Labour leader is that he had inherited a pro-EU party.

Ever since Jacques Delors, then president of the European Commission, infuriated Margaret Thatcher by promising left-wingers a 'social Europe' at the 1988 Trades Union Congress (TUC) conference, the Labour Party had been on board with the EU project.

Corbyn, who voted to quit the European Economic Community (EEC) in 1975, would use a Delors-style argument to defend his new "warts and all" stance when he made his first speech of referendum campaign on 14 April.

"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," he argued.

Corbyn's pitch to Labour voters was simple enough: We need to stay in the club, to reform it. But most of the University of London speech centred on anti-Conservative rhetoric – the junior doctors' dispute and tax evasion were raised – rather than a blistering defence of the EU.

Dave Nellist, the chairman of the Trade Unionist and Socialist Coalition (TUSC), was unconvinced. "What Jeremy did last summer – in enthusing a new layer of people getting involved with Labour – was on the basis of challenging the status quo and challenging the establishment on austerity, on tuition fees on the re-nationalisation of rail and energy" the left-winger told IBTimes UK.

"He's backing off on those things. When it comes to the re-nationalisation of rail and steel, one of the big problems with the EU is that the block it puts on any government carrying out a programme of nationalisation or re-nationalisation – that's what Bob and Tony stood for."

However, others on the left are adopting a similar position to Corbyn. The UK's two largest trade unions, Unite and Unison, are backing a 'remain' vote as well as the TUC and the GMB union. Another left-wing Dave, Dave Prentis, has claimed staying in the 28-nation bloc will safeguard millions of public service jobs.

"Europe isn't perfect, but on balance staying in the EU has so much more to offer nurses, teaching assistants, town hall staff and other public servants than an uncertain future where the UK goes it alone," the Unison general secretary declared.

"Brexit fears have just seen the UK's growth forecast downgraded. NHS, local government, school and police employees bore the brunt of the last economic downturn, and are still paying the price. The last thing anyone wants is another recession where jobs, living standards and public services are back on the line."

However, that pro-EU stance is not shared by rail unions the RMT and Labour affiliated Aslef, who are backing a 'leave' vote on 23 June. Corbyn reminded them that it was Conservative Prime Minister John Major who drove rail privatisation through in the 1990s.

The unions aren't budging. Maybe they agree with Nellist? That Corbyn "is a prisoner of a right-wing parliamentary Labour Party."