Tony Blair has urged pro-Europeans to "rise up" and persuade the British people they were wrong about Brexit.
In a keynote speech, the former prime minister said voters had backed leaving the EU without knowing the true cost and should have the opportunity to change their minds.
Speaking at the headquarters of the Bloomberg financial news agency in London where David Cameron first set out his plan for an in/out vote on Britain's EU membership, he questioned whether the referendum had given a mandate for "Brexit at any cost".
He warned that the path the Government was now pursuing meant the break-up of the UK was "back on the table", giving the SNP a much more credible case for Scottish independence.
"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," he said.
"What was unfortunately only dim in our sight before the referendum is now in plain sight. The road we're going down is not simply hard Brexit. It is Brexit at any cost.
"Our challenge is to expose relentlessly what this cost is, to show how the decision was based on imperfect knowledge which will now become informed knowledge, to calculate in 'easy to understand' ways how proceeding will cause real damage to our country; and to build support for finding a way out from the present rush over the cliff's edge.
"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."
Mr Blair said that in the absence of an effective opposition, pro-Europeans needed to build a "movement " that reached across party lines.
He said that the institute which he was launching would play its part in developing the arguments to rethink the country's position.
"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.
"These groups must find ways of concerting strategy and tactics effectively. We should begin to create informal links immediately and then build them into a movement with weight and reach.
"We need to strengthen the hand of the MPs who are with us and let those against know they have serious opposition to Brexit at any cost."
Mr Blair said Brexiteers had been the beneficiaries of a "propensity for revolt" which characterised the current state of politics, but that did not mean voters' views on leaving the EU were set in stone.
"They will say the will of the people can't alter. It can. They will say leaving is inevitable. It isn't. They will say we don't represent the people. We do, many millions of them and with determination many millions more," he said.
"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."
This time last year, only a few of us were suggesting that Brexit was likely. The mainstream view was that it couldn't possibly happen. But it did. And so too did Trump. When this column argued in June that "we should prepare for President Trump", one or two local talk shows chuckled and sneered at the mere suggestion that such a creature could inhabit the White House. But he is there.