Tony Blair has told Remain voters "we're the insurgents now" and urged them to persuade Brexit supporters that the country is better off staying in the European Union.
The former prime minister said the will of the people should be respected but insisted Remain voters "have to believe in the people's innate sense, that they're also open to a better argument in the light of the facts as they come to light".
Issuing his rallying cry, he said Remain voters should mobilise and organise to "prise apart the alliance which gave us Brexit" and insisted that, as the "catastrophic" realities of leaving take hold, staying in must remain an option.
He highlighted the sharp fall in the pound as a "negative prediction about our economic future", and the blocking of a Canada-EU free trade deal by the Walloon regional parliament in Belgium as evidence that Brexit may not take the form many Leave voters had imagined.
Brexit-backing politicians are now openly acknowledging that leaving means freeing Britain from its "essential social democratic" model, including a free NHS, he said.
In its place they envisage "free market, free trading, light regulation, low tax, low social protection" like in city states Hong Kong and Singapore - something which not all Leave voters would back.
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.
"Maybe it won't, in which case people like me will have to accept it.
"But surely we are entitled to try to persuade, to make the argument, and not to be whipped into line to support a decision we genuinely believe is a catastrophe for the country we love."
He said Remain voters must win the argument that staying in the EU should stay on the table, warning that anger and anxiety about immigration, globalisation, stagnant incomes, housing and squeezed public services will only get worse outside the EU.
"Right now there is one point and one point only to win: we should keep every option open," Mr Blair wrote.
"That this should even be contentious speaks loudly about how much those of us - and after all we were 16 million people - who believe Britain's future lies within the European partnership, have been shoved on to the defensive.
"We have to respect that people voted as they did. But we have to believe in the people's innate sense, that they're also open to a better argument in the light of the facts as they come to light.
"We 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."
He added: "This is a world which changes fast. There is a downside to that. But there is also an upside. Things which look resolved emphatically can be open to a new resolution.
"Above all stay firm. We're a sovereign people. We can make up our mind; and we can change our mind. And whether we do, is up to us."