Owen Smith has promised to attempt to block the triggering of formal negotiations to leave the European Union (EU) until the Government offers a second referendum or calls a general election to approve its final Brexit deal.
The former shadow work and pensions secretary said that under his leadership the Labour Party will vote against triggering Article 50 until the Conservatives commit to a second public vote.
Outlining his position exactly two months after the outcome of the historic vote was announced, Mr Smith warned "Labour won't give the Tories a blank cheque".
He hit out at comments made by Jeremy Corbyn on the morning of the EU referendum result calling for Article 50 to be invoked immediately. The Labour leader has since backtracked and insisted he misspoke.
Mr Smith said: "Jeremy Corbyn's decision to call for Article 50 to be invoked immediately after the referendum result was deeply irresponsible and, if he had been listened to, would have damaged Britain's interests.
"The British people were lied to by the Leave campaign - they deserve to have a say on whatever exit deal the Tories strike with the EU.
"Theresa May says that 'Brexit means Brexit' - but nobody knows what Brexit looks like.
"It could involve trashing workers' rights and environmental protections, opening our NHS up to foreign competition, making it harder for us to trade with our neighbours and damaging our economy."
He added: "I'm a passionate pro-European and I will fight tooth and nail to keep us in the EU.
"Under my leadership, Labour won't give the Tories a blank cheque. We will vote in Parliament to block any attempt to invoke Article 50 until Theresa May commits to a second referendum or a general election on whatever EU exit deal emerges at the end of the process. I hope Jeremy will support me in such a move."
Mr Smith has previously promised to offer the British public a second referendum to ratify any Brexit deal.
In order to leave the EU, the UK must trigger Article 50 of the Lisbon Treaty which gives the two sides two years to negotiate the terms of the split.