Jeremy Corbyn will say Labour is "overwhelmingly" in favour of Britain staying in the EU as he makes his first major intervention in the referendum campaign.
The Labour leader - who during the leadership contest did not rule out backing Brexit and who was accused of giving only lukewarm public support so far to the "remain" cause - will point to serious "shortcomings" in Brussels.
But he will say the Conservatives, not the EU, were responsible for the crisis in the British steel industry and a failure to tackle tax avoidance and that Labour would work with continental allies on such issues.
And he will set out what he describes as a "strong socialist case for staying in", including the protection of workers' rights, strict environmental standards and the protection of consumers from rip-off charges.
"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 say in a speech in central London.
Issues such as climate change, corporate tax dodging, cyber-crime, terrorism, protecting jobs and pay from the pressure of globalisation, the refugee crisis and ageing populations "self-evidently require international co-operation", he will say.
"Collective international action through the European Union is clearly going to be vital to meeting these challenges. Britain will be stronger if we co-operate with our neighbours in facing them together."
Mr Corbyn will acknowledge that he remains "critical of its shortcomings, from its lack of democratic accountability to the institutional pressure to deregulate or privatise public services".
"So 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."
He will add: "By working together across our continent we can develop our economies, protect social and human rights, tackle climate change and clamp down on tax dodgers.
"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."
Mr Corbyn will warn against the temptation "to blame the EU, or worse to blame foreigners" for the country's problems when the issue was "a Conservative government that is failing the people of Britain".
"There is nothing remotely patriotic about selling off our country and our national assets to the highest bidder, or in handing control of our economy to City hedge-funds and tax-dodging corporations based in offshore tax havens.
"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.
"But we also need to make the case for reform in Europe - the reform David Cameron's government has no interest in, but plenty of others across Europe do.
"That means democratic reform to make the EU more accountable to its people, economic reform to put jobs and sustainable growth at the centre of European policy, labour market reform to strengthen and extend workers' rights in a real social Europe, and new rights for governments and elected authorities to support public enterprise and halt the pressure to privatise services."
He 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.
"There are certainly problems about EU state aid rules, which need reform," he will say.
"But if, as the Leave side argues, it is the EU that is the main problem, how is it that Germany, Italy, France and Spain have all done so much better at protecting their steel industries?
"It is not the EU that is the problem, but a Conservative government here in Britain that doesn't recognise the strategic importance of steel, for our economy and for the jobs and skills in those communities.
"A Labour government would have worked with our partners across Europe to stand up for steel production in Britain."
Tory MEPs had also "voted down attempts to clamp down on tax dodging", he will say.
"Labour has allies across Europe prepared to take on this global network of the corrupt and we will work with them to clamp down on those determined to suck wealth out of our economies and the pockets of our people.
"Left to themselves, it is clear what the main Vote Leave vision is: for Britain to be the safe haven of choice for the ill-gotten gains of every dodgy oligarch, dictator, or rogue corporation.
"They believe this tiny global elite is what matters, not the rest of us, who they dismiss as 'low achievers'.