Remaining in the European Union is in the best interests of the British people, Jeremy Corbyn will say as he vows to use the referendum campaign to "stand up for public ownership".
The Labour leader will vow to push for a "real social Europe" in the run-up to the vote promised by David Cameron.
Mr Corbyn, who has a history of Euroscepticism, will stress that he wants to see further reforms in Brussels, including stronger workers' rights and an end to the pressure to privatise public services.
In a speech to the Labour local government conference in Nottingham he will say: "In the referendum campaign Labour will be making it clear we stand up for public ownership and accountability.
"Our party is committed to keeping Britain in the EU because we believe it is the best framework for European trade and co-operation and is in the best interests of the British people.
"But we also want to see progressive reform in Europe: democratisation, stronger workers' rights, sustainable growth and jobs at the heart of 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."
His intervention comes as the Labour group calling for a Brexit was in disarray, with senior figures issuing conflicting claims about its relationship with one of the main Leave campaigns.
The Labour Leave group was formally launched in January, with the MPs behind the group highlighting Mr Corbyn's previous sympathy for their cause.
But former minister Kate Hoey and the group's co-founder John Mills - who has been one of Labour's biggest donors - are at odds over the group's ties to Vote Leave, one of the two organisations seeking the Electoral Commission's designation as the main Brexit campaign.
Ms Hoey told Channel 4 News: "Labour Leave is not going to be aligned to any of the two main groups at the moment, Leave.EU and Vote Leave."
But Mr Mills, who is vice-chairman of Vote Leave, said: "Labour Leave is an independent campaign but corporately it supports Vote Leave."
Mr Mills said he was the "founder and co-owner of Labour Leave", but Ms Hoey said: "John Mills is not an office holder. He was part of the people who set it up, he was chair of Vote Leave and then during the week he was demoted to being the vice-chair of Vote Leave."
Trouble within Vote Leave, which also lost the support of Green peer Baroness Jones, appeared to centre on the role of campaign director Dominic Cummings, a former aide to Tory cabinet minister Michael Gove.
Ms Hoey acknowledged that Mr Cummings' role was one of the factors in her decision, telling Channel 4 News: "Well, I suppose his name does come into the frame."
Meanwhile, Sky News reported that the Government was trying to enlist the help of political guru Sir Lynton Crosby to help sell Mr Cameron's proposed deal amid concerns about how the reform package has been received.
The controversial Australian has been credited with playing a major part in the Tory success in the 2015 general election.