He has been criticised for not speaking-up in the debate so far...
Jeremy Corbyn will say Labour is "overwhelmingly" in favour of Britain staying in the EU - although many argue the Labour leader himself still is not.
Making his first major intervention in the referendum campaign, Mr Corbyn will point to serious "shortcomings" in Brussels and say he is "making the case for 'Remain and Reform' in Europe."
The Labour Leader has been criticised for mostly sitting out of the debate until now.
But he will say in a speech in central London: "The Labour Party is overwhelmingly for staying in, because we believe the European Union has brought investment, jobs and protection for workers, consumers and the environment, and offers the best chance of meeting the challenges we face in the 21st century."
Mr Corbyn will add: "There is a strong socialist case for staying in the European Union, just as there is also a powerful socialist case for reform and progressive change in Europe."
The speech comes as a YouGov poll in The Times (London) has Leave and Remain tied on 39% - with 17% of voters unsure of which side they will back.
During the leadership contest Mr Corbyn did not rule out backing Brexit, and Labour Leave campaigner Graham Stringer believes he had "honourable and pragmatic reasons for changing his position" in order to keep the mostly pro-EU Parliamentary Labour Party together.
The MP for Blackley and Broughton told Sky News: "I don't believe deep down, apart from his responsibility as leader of the Labour Party, he has really changed his position. He understands the European Union is undemocratic, almost anti-democratic."
Alan Johnson, a former home secretary and chairman of the Labour 'In' campaign, said the party may be divided on other issues, but it was united when it comes to Europe.
He told Sky News: "It (the EU) is not perfect, no institution is. The United Nations isn't, the Houses of Parliament aren't perfect.
"But it's a very different argument saying that things need to change and be reformed to saying we should walk away from it."
Mr Corbyn will acknowledge that he remains critical of the EU's lack of democratic accountability, but will argue that "working together across our continent we can develop our economies, protect social and human rights, tackle climate change and clamp down on tax dodgers."
Ayesha Hazarika, a former adviser to Ed Miliband, described this as a "good and honest message."
She said: "There is a big group of people who are Labour supporters who want to stay in Europe but want a more progressive Europe, they want a Europe that works for working people, not just the bosses."
Mr Corbyn will dismiss claims by pro-Brexit campaigners that the EU was to blame for the collapse of the UK steel industry - instead accusing the Government of blocking proposed tariffs on Chinese imports.
But he sets out a long wish-list of changes including: