Jeremy Corbyn insisted there was "nothing half-hearted" about Labour's pro-EU campaign as he made his first major speech of the referendum battle.
The party leader — who voted Out in the 1975 referendum and has expressed Eurosceptic views over subsequent decades — has been accused of making only a lukewarm contribution to the Remain argument so far.
But he said it was clear that the party was “overwhelmingly convinced” that being part of the bloc was in the best interests of the country on issues such as workers’ rights and the environment.
There remained serious "shortcomings" that needed to be addressed by Brussels, such as the proposed trade deal with the US which gave "huge cause for concern" about the potential for privatisation of public services, he warned.
All of those could be better dealt with, however, by remaining in the EU “warts and all” rather than by pulling out and leaving the country at the mercy of the Conservatives, he argued.
"We have had a very big debate within the party and within the trade unions. Overwhelmingly, the Labour Party and the trade unions have come to the view that they want to campaign for a social, just Europe to protect the workers' rights that we've got, to extend them and extend that degree of justice.
“That is the position we have reached. That is the position that has been adopted by the party. That is the party that I lead and that is the position I am putting forward.”
He went on: “There is nothing half-hearted about what we are doing, there is nothing half- hearted about our campaign, there is nothing half-hearted about our alliances.
“I have attended a number of meetings of the Party of European Socialists, I have had lengthy conversations with prime ministers and party leaders all across Europe on the social justice case, the environmental case, the issues of climate change, trade and steel and all those issues.
“I have made numerous speeches on all these subjects. There is nothing half-hearted about what we are doing.”
‘Shortcomings’
Mr Corbyn said: “Over the years I and many others have been critical of many decisions taken by the EU.
“And I remain very 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 European Union.
“It’s perfectly possible to be critical and still be convinced we need to remain a member,” he said, suggesting his own election as Labour leader — after years at loggerheads with the direction of the party — showed what could be achieved.
“I’ve had a few differences with the direction the Labour Party’s taken over the past few years as some people may have noticed,” he joked.
“But I have been sure that it was right to stay as a member of the party. I joined the Labour Party when I was 16 and I am very proud of that.
“Some might say I’ve even managed to do something more recently about changing the direction of the Labour Party and I’m enjoying that as well.
“In contrast to four decades ago, the EU of today brings together most of the countries of Europe and has developed important employment, environmental and consumer protections.
“I have listened very closely to the views of trade unions, environmental groups, human rights organisations and of course to Labour Party members and supporters and fellow MPs.
“They are overwhelmingly convinced that we can best make a positive difference by remaining in Europe.
“Britain needs to stay in the EU as the best framework for trade, manufacturing and co-operation in 21st century 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.”
He said he did not believe “too many” EU nationals had come to live and work in Britain — and said higher wages, not curbs on free movement, were the key to immigration questions.
“It is employers that try to undercut industry-wide agreements in the construction industry and others that are the problem,” he said.
“There is nothing wrong with people migrating to work around the continent but there has to be a level playing field on pay and conditions.”
Jobs and rights
He said he did not “recant on everything I’ve ever said and done” on the EU issue but was fighting for the chance to change it from within.
TUC general secretary Frances O'Grady said: "The Labour leader is absolutely right to warn that Brexit is a danger to workers' jobs and rights.
“The broader labour movement is united behind the benefits of EU membership for working people.
“Many of the biggest cheerleaders for Brexit have spent years dismissing rights like paid leave and maternity protections as ‘red tape’ to be binned.
“So we know from their hostility that the threat to workers’ rights if we leave the EU is very real.”
Mr Corbyn dismissed 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.
“The Conservative Government has blocked action on Chinese steel dumping. It has cut investment in infrastructure that would have created demand for more steel and had no procurement strategy to support British steel,” he said.
“The European Union — 28 countries and 520 million people — could have made us stronger, by defending our steel industries together. The actions of the Conservative Government weakened us.”
He also accused David Cameron and Tory MEPs of seeking to “undermine” EU efforts to expose tax avoidance.
“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,” he said.
“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’.”