Tony Blair: Options must stay open on Brexit
The UK should keep its "options open" over Brexit, Tony Blair has said, adding that a second referendum should not be ruled out.
The former PM told the BBC the vote was a "catastrophe" and said it was vital to study the "real-life implications".
Mr Blair said he accepted the verdict of June's referendum, but recommended looking again at Brexit when "we have a clear sense of where we're going".
Downing Street said it was "absolutely committed" to seeing Brexit through.
A spokesman said the British people had expressed their view very clearly on 23 June, when 51.9% of voters opted to leave the EU, adding: "There will be no second referendum."
Supporters of leaving the EU argue it will free up the UK to trade better globally and give the government better control of immigration.
'We're the insurgents now'
But Mr Blair told BBC Radio 4's Today programme he believed the Brexit vote was a catastrophe and argued that it was important that the views of the "16 million" people who had backed remaining in the EU should not be ignored.
He added: "If it becomes clear that this is either a deal that doesn't make it worth our while leaving, or alternatively a deal that's going to be so serious in its implications people may decide they don't want to go, there's got to be some way, either through Parliament, or an election, or possibly through another referendum, in which people express their view."
But he said the vote for Brexit could not be changed "unless it becomes clear that the British people have had a change of mind".
Mr Blair's interview followed an article in the New European newspaper in which he said those who believed in the EU "have to recognise we're the insurgents now".
"We have to build the capability to mobilise and to organise. We have to prise apart the alliance which gave us Brexit."
UKIP MP Douglas Carswell tweeted that Mr Blair was "seeking to de-legitimise and reverse" the referendum result.
The government has promised to invoke Article 50 - setting formal talks with the EU in motion - by the end of March next year.
It says it will not provide a "running commentary" on its stance before negotiations for leaving the EU begin but has pledged to make Brexit work for the whole country.
Mr Blair told the BBC it was unclear what the outcome of negotiations, in terms of restrictions of movement of people and access to the single market, would be, adding: "We will start to see the real-life implications of this decision to go."
'Very, very tough'
"We've got to work out: are the freedoms that we are going to gain really so substantial that we want to leave the European Union?"
He warned of the talks with the EU: "I'm convinced that it's going to be very, very tough. We have to understand we are not going to be conducting these negotiations with a group of European businessmen who might well decide that they want maximum access to the UK...
"The people we are going to be conducting these negotiations with are the political leaders of the European Union and their parliaments.
"I'm arguing we should keep our options open," he said.
Mr Blair, who was prime minister between 1997 and 2007, announced last month that he was winding up most of his commercial ventures to focus on not-for-profit work.
In 2004, Mr Blair said a UK referendum would be held on ratification of the European Constitution Treaty, creating, among other measures, a European Council president and a "common defence policy". But voters in France and the Netherlands rejected the treaty, which was discarded.
In 2007, the Labour government rejected calls for a referendum on a second treaty, which came to be known as the Lisbon Treaty, arguing it was a different document and a UK referendum was not needed.
Earlier this month, Mr Blair told Esquire magazine he was considering whether there was a "role" for him in UK politics, having not been actively involved for almost a decade.