Labour votes will be crucial if the Remain side is to win the referendum on EU membership, Jeremy Corbyn has said as he launched the party's campaign.
He appealed to the 9.3 million people who voted Labour at the last general election to turn out on 23 June, saying many of their jobs are dependent on trade with Europe.
The Labour leader has been urged to make a bolder case for EU membership.
Apart from one major speech, Mr Corbyn has kept a low profile on the issue.
While all but a handful of Labour MPs support staying in the EU, Mr Corbyn is regarded as the most Eurosceptic leader of the party in a generation. He has called for a more "social Europe" focused on workers rights and social solidarity.
Ahead of the launch of Labour's pro-EU campaign battle bus, Mr Corbyn told The Times the UK faced a "historic choice" on 23 June and he believed the UK would be stronger to face the challenges of the 21st Century "together".
At the launch, Mr Corbyn said despite his "many criticisms" of the EU, he was fully behind the Remain campaign because membership had helped to "protect" workers' rights and the environment.
"I believe we have to vote to Remain in order to defend investment, to defend jobs, to defend the environment, to defend workers' rights," he said.
"We see it as an act of solidarity with other people that think like us across Europe.
"Going it alone won't help them and won't help us."
The Labour leader also said immigration did "not necessarily" put a strain on public services or have a negative impact on wages.
'Out of touch'
Labour In, led by former Home Secretary Alan Johnson, is embarking on a grassroots campaign targeting students, factory workers and young parents across the country, with a campaign bus visiting 100 towns and cities across the country.
Mr Johnson said being in the EU and having access to the single market was "crucial" to the UK's prosperity.
"To pretend that it isn't and we can wander off into a sepia-tinted world of the 1950s is just a complete abrogation of responsibility," he said.
Mr Johnson also disputed claims by Leave campaigners that the UK sent £350m a week to Brussels.
But even if that same amount was redirected to the NHS, it would be a "drop in the ocean", he added.
Fellow Labour MP Gisela Stuart, who chairs the Vote Leave campaign group, said his remarks showed the Remain campaign was "out of touch".
"The Remain campaign wants the elites to have more power and money, and not to give back control to the British people," she said.
"Patients struggling to get care on the NHS will rightly think that they are completely out of touch when they claim that £350m - enough to build a new hospital every week - is just a drop in the ocean."
The majority of trade unions are recommending their members vote to remain in the EU although Labour Leave, whose members include Ms Stuart and Frank Field, say leaving the EU will allow more money to be spent on the NHS and other public services.