Owen Smith has promised to attempt to block formal negotiations with European Union leaders until the government offers a second EU 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 on membership of the EU.
Mr Smith criticised 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.
"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," said Mr Smith.
"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.
"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."