Owen Smith has said he could apply to take the UK back into the European Union if he was prime minister - and potentially sign the country up to the euro and the Schengen open borders zone.
The Labour leadership hopeful said if the country had broken from Brussels by the time he takes office he would be prepared to reapply to join the EU.
Mr Smith said he would fight a general election making a “really strong case for us to stay” in the EU, but if the country had already left then “hypothetically” he could apply for the UK to rejoin.
Theresa May has said she will not trigger Article 50 - which begins the formal two-year exit period - before the end of the year, but if she were to take the step next year the UK would leave in 2019, before the next scheduled general election.
Although formal negotiations on the Brexit terms cannot be thrashed out until Article 50 is invoked, Mr Smith disputed that and said Mrs May must reveal her plan.
“If she were to trigger Article 50 before the British public knows what the real Brexit deal is, I think that would be dereliction of duty on her part,” he told BBC’s Andrew Marr Show.
“My point is simple - once we know what the deal is, that’s the point at which you have an extra democratic moment in Britain.”
Asked if he would try to regain entry into the EU after Brexit if he was in Number 10 following the 2020 general election, Mr Smith said: “I think it’s very hard to answer because it’s a hypothetical question ... If we had gone into a further recession, if we had the prospect of another 10 years of Tory austerity, if they were saying that the price of our staying out is opening up the NHS to private sector competition, it is worse terms and conditions - more flexibility and less red tape as the Tories would no doubt dub it - then I think the sensible and responsible thing for a Labour government to do is to say we are better off in the European Union.”
Asked whether, as a new member, the UK could be obliged to sign up to the single currency and the Schengen Area, Mr Smith said: “Potentially, but again we are getting into hypotheticals built on hypotheticals.
“The key thing is they have not said definitively - a couple of weeks ago they were briefing it might be 2019 before they trigger Article 50.”
Mr Smith said he was an internationalist and a collaborator and a co-operator” adding: “I want us to be part of the European Union”.
Many of Labour’s former industrial heartlands voted enthusiastically for Brexit but Mr Smith insisted that they could be persuaded to back his policy.
“If we have gone into a further recession, if the NHS is on its knees as it is right now, if we have got the prospect of more Tory austerity, then I think we will be telling a very different story to the British people.
“I think people in those Northern cities don’t want more reductions in their livelihoods, they want to see investment in their communities and Labour at that point - at some point in the future - would make a really strong case for us to stay within the EU.”
Jeremy Corbyn remains the clear favourite to win the leadership contest when the result is announced on September 24, but Mr Smith said: “You can’t mistake mass rallies for a mass movement.”
Mr Corbyn “has got mass appeal to a small section of the electorate”, the leadership challenger said.
Asked how confident of success he was on a scale of one to 10, Mr Smith said: “Ten, absolutely. Never answer seven-and-a-half, always say 10.”
Mr Corbyn had suggested he was “seven-and-a-half” when asked how enthusiastic he was about staying in the EU during the referendum campaign.
The leadership challenger said he offered a credible alternative to the plans set out by Mr Corbyn and shadow chancellor John McDonnell.
Mr Smith’s plans would involve an extra £200 billion of borrowing along with tax hikes for the rich - raising the top rate of income tax to 50p and introducing a wealth tax.
“I think credibility is our problem. I have said explicitly how we would raise that money and no economist in the country has disagreed with me about the ability of the British government to issue long-dated debt.
“But John McDonnell has said he wants to raise £500 billion, he says he is only going to do that through increasing the productivity of the economy and getting tax avoidance down.
“Tax avoidance will raise maybe £30, £40 billion if we are lucky.”
Mr Corbyn put education reform at the heart of his latest push for votes, vowing to overturn the Prime Minister’s plans for a new generation of grammar schools in England.
Writing in the Sunday Mirror, he said: “Theresa May has no mandate to make these changes. Therefore, if these divisive plans go ahead, I will make it a top priority to reverse them when Labour is back in power.”