Mr Smith has claimed that it was “hypothetically” possible for the UK to rejoin the EU, even if that means signing up to the euro and Schengen area.

The MP for Pontypridd, who has constantly called for a second referendum, speaking on The Andrew Marr Show said he would consider the move if Britain had entered a “further recession” and if he became PM.

The 46-year-old said: “I think at that point if we have gone into a further recession, if we had the prospect of another 10 years of Tory austerity, if they were saying the price of us staying out is opening up the NHS to private sector competition and there are worse terms and conditions, more ‘flexibility and less red tape’, as the Tories will no doubt dub it.

“Then I think the sensible and responsible thing for a Labour government to do is say, ‘we’re better off in the European Union’.”

Mr Smith, when challenged by Marr, said he would “potentially” agree to scrap the pound in favour of the euro and join the Schengen area if it means Britain can rejoin the EU.

He added: “The key thing is they’ve [the Conservatives] not said definitively [when we will leave] – a couple of weeks ago they were briefing it might not be until 2019 because they trigger Article 50.

“I’m an internationalist, a collaborator and a cooperator, I want Britain to be part of the European Union.”

Mr Smith’s comments drew anger on social media.

On Twitter, Rrose SelaKlee said: “#marr Owen Smith will take us back into th EU if he gets a chance, with Euro & free mvmnt….Last slim hope for Labour has just disappeared.”

Naomi55 added: “Did Owen smith just say he would reapply to the EU if we are out?!?! How utterly useless is this guy?”

This comes shortly after senior Conservative and Brexiteer John Whittingdale hit out at Prime Minister Theresa May calling on her to invoke Article 50 “in weeks” to stop Remain backers from blocking Brexit.

He said: “The Prime Minister has been admirably clear in saying that Britain has voted to leave, so we are going to leave but until you embark on the formal process there will be some who will continue to suggest that somehow this can be fudged, that we can make some changes but still essentially remain a part of it and will attempt to find a way out.”

Mr Whittingdale, who was removed from cabinet during Ms May’s cabinet reshuffle, said leaving the Union must happen much sooner than the Tory leader’s suggested timeline.

The Prime Minister claimed she would not invoke Article 50, beginning the formal process of leaving the EU, until next year at the earliest.

Foreign Secretary Boris Johnson is also mounting pressure on the Prime Minister as he leads a group demanding “hard Brexit”.

Spearheading the Change Britain campaign – which is made up of Brexiteers Michael Gove, Graham Brady and David Cameron’s former advisor Steve Hilton – he ordered Ms May to deliver on the Conservative party’s promises.

In a video promoting the group, Mr Johnson said: “On June 23 the people of this country voted to leave the European Union and they voted for change.

“They did so by a clear majority. But there were many people who also voted for Remain.

“So it’s absolutely vital that we work together, Leavers and Remainers, as we seize the opportunities that this country now has to forge a positive and exciting new relationship not just with the European Union but also with the rest of the world; changing Britain and making it global again.

“No more than ever we need to show the British people that as politicians we are listening what they have to say.”