Jeremy Corbyn has announced that he will demand a second EU referendum when the next Prime Minister is in post.
In his firmest stance yet, the Labour leader said his party will officially campaign to remain if a second vote is called.
In a letter to Labour members, Mr Corbyn said he accepted the 2016 referendum result, but that the possibility of a no-deal Brexit led to his decision.
“Both Tory leadership candidates are threatening a no-deal Brexit - or at best a race to the bottom and a sweetheart deal with Donald Trump,” he said.
“Whoever becomes the new Prime Minister should have the confidence to put their deal, or no deal, back to the people in a public vote.”
He added: “In those circumstances, I want to make it clear that Labour would campaign for remain against either no deal or a Tory deal that does not protect the economy and jobs.”
Labour had previously only backed a second referendum in specific circumstances, and faced criticism from party members for avoiding a more resolute position.
Mr Corbyn said the new Conservative leader must also call a general election.
“Our country is ravaged by inequality and rising poverty, huge regional imbalances of investment, and the government is failing to tackle the climate emergency facing us all,” he said.
“We need a general election. After nine years of austerity, too many people in this country cannot find decent secure well-paid work, and have to rely on public services that have been severely cut back.”