Tony Blair will tell pro-EU Britons it is their "mission" to persuade Brexit voters to change their mind about leaving the European Union.
In a speech on Friday, the former prime minister will urge Remain voters to "expose relentlessly" the Government's drive for a "Brexit at any cost", which will cause "real damage" to Britain and the embitterment of future generations.
In a speech for Open Britain, which is campaigning against a so-called "hard Brexit" outside the European single market, Mr Blair will issue a rallying cry against the referendum vote which was "based on imperfect knowledge".
The Labour former PM is expected to say: "The people voted without knowledge of the true terms of Brexit. As these terms become clear, it is their right to change their mind.
Mr Blair will say Theresa May is leading a "mono-purpose" Government "for Brexit, of Brexit and dominated by Brexit".
"Those driving this always wanted a hard Brexit. Indeed even the term hard Brexit requires amendment. The policy is now Brexit at any cost.
"Our challenge is to expose relentlessly the actual cost, to show how this 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 the country and its citizens 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."
He will rail against the Prime Minister's characterisation of a pro-EU metropolitan elite who are "citizens of nowhere", insisting: "This is not the time for retreat, indifference or despair; but the time to rise up in defence of what we believe.
"How hideously, in this debate, is the mantle of patriotism abused. We do not argue for Britain in Europe because we are citizens of nowhere. We argue for it precisely because we are proud citizens of our country who believe that in the 21st Century, we should maintain our partnership with the biggest political union and largest commercial market right on our doorstep; not in diminution of our national interest, but in satisfaction of it."
Mr Blair will acknowledge "genuine concerns" over immigration but claim that "for many people" the main worry is around entrants from countries outside the EU, "especially when from different cultures in which assimilation and potential security threats can be an issue".
He will say: "Nonetheless, we have moved in a few months from a debate about what sort of Brexit involving a balanced consideration of all the different possibilities; to the primacy of one consideration - namely controlling immigration from the EU - without any real discussion as to why and when Brexit doesn't affect the immigration people most care about.
"Yet we're told we have to stop debating it and just do it.
"This is a great country, with resilient and creative people. And yes, no one is going to write us off. But making the best of a bad job doesn't alter the fact that it isn't wise to put yourself in that position unless you have to."
Mr Blair will warn that the break-up of the UK is now "back on the table" but with a "much more credible" case for Scottish independence.
Reflecting on his own legacy, he will also highlight the "destabilising impact" of concerns over border arrangements on the Northern Ireland peace process.
"None of this ignores the challenges the country faces in common with many other countries: those left behind by globalisation; the aftermath of the financial crisis; stagnant incomes amongst a section of the public; and for sure the pressures posed by big increases in migration which make perfectly reasonable people anxious and feeling unheard in their anxiety," he will say.