Tony Blair has called on pro-Europeans to form a new cross-party movement to persuade the public they were wrong to vote to leave the EU.
In an impassioned speech, the former prime minister said "progressives" should make it their "mission" to reverse the outcome of the referendum last June.
Speaking at the London headquarters of Bloomberg, where David Cameron first set out his plan for an in/out vote on Britain's EU membership, Mr Blair rejected the idea that leaving was now "inevitable".
He said that when people had voted last year they had not understood the full cost of withdrawal - including the potential break-up of the UK, with Scottish independence "back on the table" and with renewed credibility.
The former PM said: "I accept right now there is no widespread appetite to re-think. But the people voted without knowledge of the true terms of Brexit. As these terms become clear, it is their right to change their mind. Our mission is to persuade them to do so.
"I don't know if we can succeed. But I do know we will suffer a rancorous verdict from future generations if we do not try.
"This is not the time for retreat, indifference or despair; but the time to rise up in defence of what we believe - calmly, patiently, winning the argument by the force of argument; but without fear and with the conviction we act in the true interests of Britain."
Foreign Secretary Boris Johnson - who led the official Vote Leave campaign in the referendum - dismissed Mr Blair's "condescending campaign" to reverse the will of the people, insisting Brexit would be "a spectacular success".
In a clear swipe at Mr Johnson and his campaign promise to release £350 million a week for the NHS, Mr Blair denounced hardline Leave campaigners as "ideologues" pursuing "a Brexit at any cost", which would leave millions worse off.
They wanted Britain, he said, to become a "low tax, light regulation, offshore free market hub" which was the "exact opposite" of the "fairer capitalism" with a better deal for workers which voters had been promised.
"The ideologues are the ones driving this bus. This free market vision would require major re-structuring of the British economy and its tax and welfare system," he said.
"It will not mean more money for the NHS, but less; actually it probably means a wholesale rebalancing of our healthcare towards one based on private as much as public provision."
Mr Blair said that in the absence of any effective opposition, the pro-Europeans would have to form a cross-party movement to build support for Britain staying in the EU.
"The debilitation of the Labour Party is the facilitator of Brexit. I hate to say that, but it is true. What this means is that we have to build a movement which stretches across party lines; and devise new ways of communication," he said.
He rejected accusations by pro-Brexiteers that in seeking to reverse the referendum result, he was simply ignoring the will of the people.
"This is the beginning in the debate. I'm not claiming any special knowledge of the British people, I'm simply claiming one right, not just for myself but for others, and that's to carry on talking to them and carry on debating with them and carry on discussing with them," he said.
Mr Johnson accused the former prime minister of "insulting the intelligence of the electorate" in arguing they had voted the wrong way.
"This is the guy who would have taken our country into the euro with what would have been catastrophic consequences," he told Sky News.
"This is the guy who dragooned the United Kingdom into the Iraq War on a completely false prospectus with consequences which foreign ministers here are still trying to deal with.
"I urge the British people to rise up and turn off the TV next time Blair comes on with his condescending campaign."
A Labour source said: " No wonder we are still trying to recover from Tony Blair's legacy when he has such contempt for democracy.
"What he doesn't seem to realise is people voted Leave precisely because they felt let down by 13 years of the Davos leftism he is still trying to flog."