Voters must be given the chance to change their minds on Brexit, Labour former prime minister Tony Blair has insisted.
Mr Blair said people need to be allowed to give their view on any Brexit divorce deal with the EU, either through a referendum, or general election.
"There is absolutely no reason why we should close off any options. You can't change this decision, unless it becomes clear in one way or another, that the British people have had a change of mind because they have seen the reality of the alternative.
"We are entitled to carry on scrutinising, and, yes, if necessary, to change our minds, because it seems sensible to us to do so. This is not about an elite over-ruling the people.
This is not about an elite over-ruling the people
"If it becomes clear that this is either a deal that doesn't make it worth our while leaving, or, alternatively, a deal that is going to be so serious in its implications that people decide they don't want to go, there's got to be some way way, either through parliament, or through an election, possibly through another referendum, in which people express their view.
"The bizarre thing about this referendum is that we, obviously, took the decision we wanted to leave the European Union, but we don't yet know the precise terms of our new relationship with Europe. Once we get clarity on that then we can decide between two alternatives, and not simply decide what we don't like about Europe," he told BBC Radio Four's Today programme.
Mr Blair, who described Brexit as "catastrophe", said that talks with French president Francois Hollande this week had given him concerns about the looming negotiations.
"It convinced me that it's going to be very, very tough. We have to understand that we are not going to be conducting this negotiation with a group of European businessmen who may well decide that what they want is the maximum access into the UK, and they may be prepared to be quite forgiving, as it were.
"The people we will be conducting this negotiation with will be the political leaders of the European Union, and their parliaments, so this is going to be a negotiation, in my view, of enormous complexity.
"If you want access into the single market you are going to have to accept obligations, and if you don't have access to the single market, then the economic costs are going to be severe," Mr Blair said.
Writing in the New European newspaper, Mr Blair said: "The issue is not whether we ignore the will of the people, but whether, as information becomes available, and facts take the place of claims, the 'will' of the people shifts."
There will be no second referendum, Britain is leaving the European Union
Tory MP Maria Caulfield, who sits on the Commons Brexit Committee, said: "Tony Blairbroke his own promise of a referendum on the EU, encouraged uncontrolled immigration and now can't come to terms with the decision of the people of the UK to leave.
"Instead of desperately trying to find ways to thwart the will of voters and talking down Britain's prospects, Labour should be concentrating on helping to make a success of Brexit."
Downing Street insisted there would be no second referendum and brushed off concerns about Mr Blair communicating his views with Mr Hollande.
A No 10 spokesman told a Westminster briefing: "Tony Blair is entitled to put his views to whom he so chooses.
"But what's important is the PM has been absolutely clear - the British people have spoken, we are listening, we're going to leave the European Union.
"And not only has the PM been clear here but she's also been clear when she's met European leaders.
"There will be no second referendum, Britain is leaving the European Union."