The Labour Party is campaigning to stay in the European Union in June's referendum, with the vast majority of the party's MPs backing continued membership.
Their leader Jeremy Corbyn is to make his first major intervention in the debate, making the "socialist" case for staying in to protect workers' rights and environmental safeguards.
It comes after much speculation about the views of a man who voted to leave the European Economic Community in the 1975.
Mr Corbyn will say he remains critical of the EU's "shortcomings", saying it is "perfectly possible to be critical and still be convinced we need to remain a member".
Leave campaigners are not convinced, saying he has been forced to "whitewash" his long-held Eurosceptic attitudes - while on the other side, some pro-EU Labour MPs have been calling for him to take a more high-profile role in the campaign.
So just how much does Mr Corbyn share his party's enthusiasm for the UK's relationship with Brussels?
Mr Corbyn has expressed Eurosceptic arguments in the past - in 1993, he spoke out against the Maastricht Treaty which established the European Union and moved towards economic and political union.
The treaty, Mr Corbyn said, "takes away from national parliaments the power to set economic policy and hands it over to an unelected set of bankers who will impose the economic policies of price stability, deflation and high unemployment throughout the European Community".
He voted against the Lisbon Treaty in 2008, and in one article on his website, said the EU had "always suffered a serious democratic deficit".
Apparently written amid speculation on who would become the first European Council president in 2009, the article said: "The creation of the post of president is a triumph for the tenacity of the European long-sighters.
"The project has always been to create a huge free-market Europe, with ever-limiting powers for national parliaments and an increasingly powerful common foreign and security policy."
Labour leadership campaign
During the Labour leadership contest last year, the candidates were regularly asked how they would campaign in the referendum, which we now know is being held on 23 June.
At a GMB hustings Mr Corbyn said "I would advocate a No vote if we are going to get an imposition of free market policies across Europe", before going on to criticise the "growing military links" with Nato and calling for trade union "harmonisation" across the bloc, "rather than just allowing it as a business free-for-all across Europe".
In another debate, hosted by the Fabian Society, he said he had "mixed feelings" on the EU, and at a hustings in Warrington said he would not rule out campaigning to leave.
He said: "I think we should be making demands: universal workers' rights, universal environmental protection, end the race to the bottom on corporate taxation, end the race to the bottom in wage protection.
"And I think we should be making those demands and negotiating on those demands rather than saying blankly we're going to support whatever Cameron comes out with in one, two years' time, whenever he finally decides to hold this referendum."
After winning a landslide victory in Labour's leadership contest, Mr Corbyn said David Cameron should not have a "blank cheque" in his negotiations ahead of the referendum.
Under pressure from MPs to clarify his position, Mr Corbyn said he wanted the UK to remain a member but would try to reverse any "damaging changes" negotiated by Mr Cameron.
"Labour is clear that we should remain in the EU. But we too want to see reform," he wrote in the Financial Times.
He added: "If Mr Cameron fails to deliver a good package or one that reduces the social gains we have previously won in Europe, he needs to understand that Labour will renegotiate to restore our rights and promote a socially progressive Europe."
The campaign gets under way
After David Cameron returned from the Brussels summit, the Labour leader said the PM's reforms were a "great opportunity missed'', adding that EU membership brought "jobs and investment".
Speaking after a meeting of the Party of European Socialists in Brussels, he said: "The Labour Party is going to be committed to campaigning to stay in the European Union.
"And when there's a Labour government in 2020, we will be trying to ensure better workers' protection across Europe, strong financial protections all across Europe, and a Europe that is based on social justice and good, rather than solely on free-market economics."
He has also stressed he is "not on the same side of the argument" as David Cameron, despite both leaders backing continued EU membership.
And he has criticised the government's £9m pro-EU leaflet drop, with his spokesman saying a "more even" assessment of the facts about Britain's EU membership would have been more useful for voters.