The Labour party backs membership of the European Union "warts and all", leader Jeremy Corbyn has said.
Mr Corbyn warned that there could be a "bonfire" of workers' rights if the UK voted in favour of a Brexit on 23 June.
Commentators have been waiting for the Labour leader to set out his stall on the all-important referendum for some time. Given his well-documented criticisms of the EU, critics have highlighted his so far lacklustre support for the 'remain' campaign.
But in this, his first major speech calling for the British public to vote to stay in the EU, Jeremy Corbyn said he stands by his previous grievances with the Union - hence the "warts and all" comment - but still believes we would be better off in.
He said: "Europe needs to change. But that change can only come from working with our allies in the EU. It's perfectly possible to be critical and still be convinced we need to remain a member."
In order to appeal directly to Labour supporters, Corbyn highlighted what a Conservative-led Great Britain might look like outside of EU oversight.
"Just imagine what the Tories would do to workers' rights here in Britain if we voted to leave the EU in June," he said.
"They'd dump rights on equal pay, working time, annual leave, for agency workers, and on maternity pay as fast as they could get away with it. It would be a bonfire of rights that Labour governments secured within the EU.
"Not only that, 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."
Prime Minister David Cameron, who spent the morning on the phones at the Britain Stronger in Europe call centre alongside former Labour leader Neil Kinnock and former Lib Dem leader Lord Ashdown, said: "I absolutely welcome Jeremy Corbyn's intervention.
"There are lots of things we disagree about, between Labour, Liberals, Greens and others, but the fact is we all come together to support the idea of Britain staying in a reformed European Union."