The former UK leader says Britons were not aware of the terms of a British exit from the EU when they voted for it in a referendum last June.
Former UK Prime Minister Tony Blair broke his silence on the British exit from the European Union on Friday, calling on fellow Britons to "rise up" against the move.
It was the first time the ex-Labour Party leader openly challenged the Brexit since the UK voted to leave the bloc in a referendum last June.
"The people voted without knowledge of the 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 told pro-European group Open Britain.
"This is not the time for retreat, indifference or despair; but the time to rise up in defence of what we believe," he said.
As TRT World's Sarah Morice in London explains, Blair, British prime minister from 1997-2007, is facing an uphill battle.
Point of no return
Blair accused the country's Conservative government of pushing for a "Brexit at any cost" following the outcome of the referendum, which he said was conducted without a clear understanding of possible consequences.
He stopped short of calling for a second EU referendum.
The result of the June 2016 referendum led then-Prime Minister David Cameron, whose government organised the vote, to step down. Although Cameron supported the referendum after pushing the EU for more favourable conditions for the UK, he campaigned to keep the UK in the bloc.
Conservative Party eurosceptics, led by current Prime Minister Theresa May, replaced Cameron and his cabinet.
May has vowed to trigger as early as March Article 50 of the Treaty on European Union, a law that sets out the process by which member states may withdraw from the bloc.
She also warned politicians against disrupting the process, which Parliament approved earlier this month.
Once May formally triggers exit negotiations, her government argues that there will be no legal way to stop the countdown to withdrawal.
The EU has not formally addressed the question, although Donald Tusk, who will oversee Brexit negotiations as chair of EU leaders' summits, did say in October that he had received legal advice that it could be stopped.
A court case in Dublin also seeks to establish whether Britain can reverse the exit process without the permission of the other 27 EU member states.
Threat to country's unity?
Blair also warned on Friday that the Brexit could lead to the break up of the UK, with Scotland again looking at the possibility of leaving.
In a referendum on secession in September 2014, Scots voted by 55-45 percent to remain part of the UK.
But the pro-European Scottish National Party has argued that the Brexit decision reopens the independence option, as most Scots voted to stay in the EU in the June vote.