Remaining in the European Union is in the best interests of the British people but there are major concerns about the proposed transatlantic trade deal, Jeremy Corbyn said.
The Labour leader, who has a history of Euroscepticism, said the Transatlantic Trade and Investment Partnership (Ttip) was being negotiated "secretly" between the EU and USA and posed a potential threat to public services.
Mr Corbyn said Labour would make the case for a "social Europe" and would stand up for public ownership during the referendum campaign.
He told activists in Nottingham: "In the referendum campaign Labour will be making it clear we stand up for public ownership and accountability and that is why Labour MEPs have been resisting the potential threats to our public services and democratic accountability by the Transatlantic Trade and Investment Partnership TTIP deal that is being negotiated rather secretly at the present time.
"Our party is committed to keeping Britain in the EU because we believe it is the best framework for European trade and cooperation and in the best interests of the people of Britain.
"But we also want to see progressive reform in Europe – democratisation, stronger workers' rights, sustainable growth and jobs at the heart of the economic policy and an end to the pressure to privatise and deregulate public services.
"And we will be pressing the case for a real social Europe during the coming referendum campaign."
Brexit campaign group Leave.EU claimed Mr Corbyn was a "lifelong opponent" of the EU but had been "gagged" by figures within Labour.
Spokesman Jack Montgomery said: "Jeremy Corbyn voted to leave the EEC in 1975 and against the Maastricht Treaty which transformed it into the EU, saying it took powers to set economic policy away from Parliament and handed them 'to an unelected set of bankers'.
"No-one really believes his private opinion has changed.
"It's extremely sad to see that Jeremy, for all his faults a conviction politician and lifelong opponent of the EU, has been gagged by the clapped out Blairites his electors rejected in the Labour leadership contest."
Meanwhile, tensions between the two rival groups campaigning for a vote to leave the EU continued to simmer.
Leave.EU's millionaire co-chairman Arron Banks identified the Vote Leave group as the "enemy", rather than the campaign to stay in the EU.
He told The Times: "The enemy for us is not the 'In' campaign — they're laughable — the enemy is our own side . . . Our job is to defeat the enemy and move on."
The rival groups are competing for the Electoral Commission's designation as the lead campaign, which would unlock a higher spending limit and a greater public profile.
Tory MP Tom Pursglove, who helped establish the Grassroots Out cross-party Brexit campaign - which has links to Leave.EU - expressed his frustration at the troubles within the Leave camp.
"One thing is for sure, it's very difficult to herd Eurosceptics," he said, as he called on Leave.EU and Vote Leave to settle their differences.
He told BBC Radio 4's Today programme: "One of the reasons we established Grassroots Out was to bring people together at grassroots level but also to try and focus the mind further up.
"People who are in the battle for designation need to sit down, sort this out, thrash it out, because every day that goes by is a missed opportunity to campaign properly in a coordinated, joined-up, together way to leave the European Union.
"I don't think that's acceptable and I think people do need to redouble their efforts to try and resolve that situation."