Jeremy Corbyn has promised a further referendum on Brexit with a “credible Leave option” if his party wins the next general election.
He said Labour was “ready” for the campaign, but its “priority” was to stop a no-deal Brexit.
The statement comes after union leaders called for a Leave option from Labour.
But some senior party figures – close Corbyn allies – say they will campaign to stay in the EU in any circumstances, even if Labour negotiates its own deal.
They include shadow chancellor John McDonnell and shadow foreign secretary Emily Thornberry, who have both said remaining would be the best thing for the UK.
Mr Corbyn has been meeting union leaders to discuss the issue.
He has not said whether he would campaign for Remain or a Labour Leave deal if he became prime minister.
The BBCs political correspondent, Iain Watson, said senior Labour figures had been arguing that backing Remain would recover ground lost to the pro-EU Liberal Democrats in recent elections.
Pro-Remain Labour activists are also hoping the party’s conference later this month will commit the leadership to backing Remain under all circumstances.
But, while Labour-affiliated unions – including Unite, who are one of the party’s biggest backers – would rather stay in the EU than have no deal, they believe a Corbyn government should offer voters a choice in a referendum between a negotiated deal and Remain.
‘Ready to unleash’
Speaking to the TUC conference in Brighton on Tuesday, he said: “Our priority is first to stop no-deal and then to trigger a general election.
“No one can trust the word of a prime minister who is threatening to break the law to force through no deal.
“So a general election is coming, but we won’t allow Johnson to dictate the terms.”
He added: “We’re ready for that election. We’re ready to unleash the biggest people-powered campaign we’ve ever seen.
“And in that election we will commit to a public vote with a credible option to leave and the option to remain.”
In a wide-ranging speech, he also promised to “put power in the hands of workers”, pledging a future Labour government would enact “the biggest extension of rights for workers that our country has ever seen.”
If elected, Labour would set up a specific government department for employment rights, he said, and give the brief to a dedicated cabinet minister.
Enforcement of rights would be boosted by a new agency with the power to enter workplaces and bring prosecutions on behalf of staff, he added.