The left-wing veteran, who has displayed a eurosceptic streak throughout his political life, said his party would make the case for a "social Europe" during the referendum campaign yesterday.
Conservative Prime Minister David Cameron wants to finalise a renegotiation of Britain's terms of membership at an EU summit on February 18 and 19.
He has committed to stage an in-or-out referendum on the outcome of his attempt, by the end of 2017.
Newspapers expect the referendum could come as soon as late June.
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"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," he told activists in Nottingham, central England.
"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."
The Leave.EU campaign group claimed Corbyn was a "lifelong opponent" of the EU but had been "gagged" by moderate, centrist figures within his party.
"Jeremy Corbyn voted to leave the EEC (European Economic Community) 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'," said spokesman Jack Montgomery.
Cameron's proposed changes include a four-year "emergency brake" limiting welfare payments to migrants, as well as a "red card" system for national parliaments to overrule draft EU laws.
A draft deal "shows real progress" but "more work" is still needed, Cameron said Tuesday.
A poll published Friday showed 45 percent of Britons now wanted to leave the EU, ahead of 36 percent who wanted Britain to stay in the bloc.
The survey of 1,675 adults was conducted by pollsters YouGov on February 3-4 for The Times newspaper.