The former Labour Prime Minister will set his sights on what he calls Ms May’s “rush over the cliff’s edge”, as she and cabinet Eurosceptics attempt to force Brexit’s launch through Parliament at breakneck speed.
Demanding the British people have a “right to change their mind”, Mr Blair will call on the 48 per cent who opposed leaving to take a stand, telling them directly: “This is not the time for retreat, indifference or despair, but the time to rise up in defence of what we believe.”
As he speaks, Ms May will be preparing for tough talks with French Prime Minister Bernard Cazeneuve, whose country’s senate has published a report concluding Britain must not be better off outside the EU after Brexit.
Mr Blair’s address in London, organised by the Open Britain campaign group, comes just days before the House of Lords starts debating legislation which, if approved, gives Ms May the right to officially begin the two-year countdown to Brexit.
Despite heralding in one of the biggest constitutional changes in British history, ministers want the bill passed unamended in a fraction of the time it normally takes to legislate.
Mr Blair will say: “This is a Government for Brexit, of Brexit and dominated by Brexit. It is a mono-purpose political entity.
“Those driving this always wanted a hard Brexit. Indeed, even the term ‘hard Brexit’ requires amendment. The policy is now ‘Brexit at any cost’.”
Mr Blair, who led Labour to three general election victories, will argue that in the 23 June referendum the British people voted without knowledge of the true terms of Brexit and that as these become clear, “it is their right to change their mind”.
Having previously argued that a second referendum or election may be needed to determine the country’s future, he will say: “Our challenge is to expose relentlessly the actual cost, to show how [the referendum] 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.”
The ex-politician has gifted more than £9m to his new policy unit for centre-ground politics, the Tony Blair Institute, with the fight against Brexit likely to form a key part of its work.
In his speech he was due to set out how the issue of immigration has been twisted by Eurosceptics to win support for the rush to the exit door.
He will say: “For many people, the core of the immigration question – and one which I fully accept is a substantial issue – is immigration from non-European countries especially when from different cultures in which assimilation and potential security threats can be an issue.
“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 Brexit doesn't affect the immigration people most care about.
“Yet we’re told we have to stop debating it and just do it.”
Mr Blair will also warn that Ms May’s drive for Brexit will strain the debate over Scottish independence and put peace in Northern Ireland at risk.
He will say: “In addition to all this, the possibility of the break-up of the UK – narrowly avoided by the result of the Scottish referendum – is now back on the table, but this time with a context much more credible for the independence case.
“We are already seeing the destabilising impact of worry 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.”
In an article for French newspaper Le Figaro today, Ms May will say Britain is not seeking to “cherry-pick” which parts of EU membership it wants to keep after Brexit, stating that her aim is to leave the single market, and strike a free trade deal including the “greatest possible access” to the trade bloc.
She will host talks with Mr Cazeneuve at Downing Street, in a bid to underline how it is in France’s interests to strike a quick and mutually beneficial deal with Britain.
But the recent report from the French Prime Minister’s compatriots in the his country’s senate suggests the divorce terms of Brexit must be completed before negotiations on Britain’s future trading relations, adding that the UK’s final agreement must not place it in a better position outside the EU.