Britain should stay in the EU "warts and all," the opposition Labour leader will say on Thursday, making his first big intervention in the referendum campaign as he seeks to counter criticism he is not doing enough to persuade his voters to back the "In" campaign.
While Labour's official position is that Britain should remain in the bloc, Jeremy Corbyn has long been eurosceptic and initially refused to rule out campaigning to leave the bloc when he was elected leader last year.
Although he has since said he backs "In," some in his party feel he has not been a vocal enough opponent of Brexit.
With opinion polls showing the contest remains tight ahead of the June 23 vote and Prime Minister David Cameron's Conservatives deeply divided over Europe, securing Labour voters' support is seen as vital to the 'In' camp winning.
"Labour is convinced that a vote to remain is in the best interests of the people of this country," Corbyn will say in a speech in London, according to extracts released by his office.
"You cannot build a better world unless you engage with the world, build allies and deliver change. The EU, warts and all, has proved itself to be a crucial international framework to do that."
Corbyn will say the EU has brought protections for workers and consumers and improved environmental standards, but insist more must be done to reform the bloc to make it more accountable to voters and boost jobs and economic growth.
Seeking to address the attention on his past views and the fact he voted "No" to Britain's membership of the forerunner to the EU in a 1975 referendum, Corbyn will say the bloc was a smaller, purely market-driven arrangement at that point.
"Over the years I have continued to be critical of many decisions taken by the EU and I remain critical of its shortcomings," he will say.
"Europe needs to change. But that change can only come from working with our allies in the EU. It's perfectly possible to be critical and still be convinced we need to remain a member."
But "Out" campaign group Leave.EU accused the Labour leader's team of "attempting to whitewash his eurosceptic past."