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 left-winger, who voted Out in the 1975 referendum and has expressed Eurosceptic views, has been accused of making only a lukewarm contribution so far.
But he said it was clear that the party was "overwhelmingly convinced" that remaining part of the bloc was in the best interests of the country.
There remained serious "shortcomings" that needed to be looked at, he warned, but added that they could be better addressed by staying in Europe "warts and all".
"Overwhelmingly, the Labour Party and the trade unions have come to the view that they want to campaign for a just Europe to protect workers' rights, to extend them and extend that degree of justice," Mr Corbyn said.
"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.
"Over the years, I and many others have been critical of many decisions taken by the EU. I remain critical of its shortcomings, from its lack of accountability to the pressure to deregulate or privatise public services.
"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.
"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. But I have been sure it was right to stay as a member of the party. I joined 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 closely to trade unions, environmental groups, human rights organisations and, of course, to Labour Party members. They are overwhelmingly convinced that we can best make a positive difference by remaining.
"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 - the reform David Cameron's Government has no interest in, but plenty of others across Europe do."