Tony Blair, the former British prime minister, has called on voters, businesses and campaigners in the UK to "rise up" and back a coordinated effort to temper the terms of, or even halt, Britain's EU exit.
Blair said on Friday that the Conservative government's drive to leave the European Union "at any cost" would hurt future generations and damage the unity of the country itself.
Last year's vote to leave the 28-nation bloc was "based on imperfect knowledge" and Britons made their decision without knowing the true terms of Brexit, he said in a speech in London.
"As these terms become clear, it is their right to change their mind," said Blair, the former Labour leader.
"Our mission is to persuade them to do so."
Blair spoke on behalf of Open Britain, which is campaigning for the government's Brexit legislation to be amended to ensure that parliament has "proper scrutiny" over any deal that Theresa May, UK prime minister, negotiates with EU leaders.
Among the group's goals is for Britain to remain part of the bloc's single market, guaranteeing unfettered access to 500 million people.
While supporters oppose any attempt to slow the drive toward Brexit, calling that undemocratic, Blair argued that the people have a right to change their minds.
The leave campaign benefited from a mood of revolt stemming in part from changes in the global economy, but such opinions are not set in stone, he said.
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"The Brexiteers were the beneficiaries of this wave. Now, they want to freeze it to a day in June 2016," he said.
"They will say the will of the people can't alter. It can. They will say leaving is inevitable. It isn't."
It was not immediately clear how much support Blair may have in creating a wave of sentiment against May's plans.
The once-popular Labour Party leader suffered a fall from grace after he supported the US in its intervention in Iraq.
'Turn off the TV'
Dismissing Blair's speech, Boris Johnson, the UK foreign minister, told the BBC that people should "rise up and turn off the TV" when Blair comes on.
Blair's intervention reflects the bitter divide that has gripped Britain since the June 23 referendum.
While 51.9 percent voted to leave the EU, the terms were not specified and May has been reluctant to discuss her plans, fearing it will hurt the UK's bargaining position.
Many in this country of 64 million have expressed disquiet at the potential consequences.
Source: News agencies