Brexit: Owen Smith opposes Article 50 move without vote
Owen Smith has said he will try to stop Theresa May triggering official EU exit talks unless she offers a referendum on the final Brexit deal or agrees to call a general election to approve it.
The Labour leadership challenger said if he is elected, the party will oppose triggering Article 50 until the government commits to a second vote.
Labour will not give the Tories a "blank cheque", he is to tell backers.
Ministers said they would deliver on the "decisive" referendum verdict.
The UK voted to leave the EU by 52% to 48% in a referendum on 23 June but the manner and timing of the country's departure remains uncertain.
The prime minister has said she will not begin the formal legal process of separation by activating Article 50 of the Lisbon Treaty this year, insisting that a "sensible and orderly departure" from the EU will take time.
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However, she has come under pressure from some senior Conservatives to begin the process in early 2017 amid press speculation about possible delays.
Speaking on Wednesday, Mr Smith will say the British people were "lied to" by those campaigning to leave the EU and they deserve to have a say on the terms of the exit.
He told BBC Radio 5 live: "It would be irresponsible of Theresa May to simply trigger Article 50 and sleepwalk out of the deal.
"Labour still believes that we should be a part of the European Union."
'Look of Brexit'
Mr Smith, who has already committed to holding a second referendum if he becomes prime minister, will say he will "fight tooth and nail" to keep the UK in the EU.
Under his leadership, he says Labour would oppose the triggering of Article 50 in a future Commons vote unless certain conditions were met.
"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 will add: "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."
Mr Smith is pressing the Labour leader Jeremy Corbyn, who he is seeking to topple, to match his commitment.
'Divided'
In the immediate aftermath of the referendum result, Mr Corbyn called for Article 50 to be invoked immediately but he has since distanced himself from the comments, saying he misspoke and due time is needed for the UK to determine its negotiating strategy.
Article 50 gives the two sides two years to negotiate the terms of the split, with any extension having to be agreed by all 28 EU members.
Sir Patrick McLoughlin, the chairman of the Conservative Party, accused Mr Smith of wanting to ignore the outcome of the referendum.
"The result of the referendum was clear and decisive and we in the Conservative Party will deliver on the people's verdict," he said.
"Whether it's Jeremy Corbyn or Owen Smith, Labour are too divided and out of touch to make a success of Brexit."
