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 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."