LONDON (Reuters) - Former British prime minister Tony Blair issued a battle cry against a so-called ‘hard Brexit’ on Friday, calling on voters, businesses and campaigners to “rise up” and back a coordinated effort to temper the terms of, or even halt, Britain’s EU exit.

In his first major political intervention since Britons voted 52 to 48 percent to leave the European Union last June, Blair said Conservative Prime Minister Theresa May was pursuing “Brexit at any cost”, and must be challenged.

“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 said in a speech.

“This is not the time for retreat, indifference or despair, but the time to rise up in defence of what we believe.”

May has vowed to start the legal process of leaving the European Union next month, and it is not clear whether the process could then be reversed. Her vision is for a clean break, including leaving the EU’s single market and customs union.

Blair, who won three elections at the head of the Labour Party, has also spoken out in the last 18 months to warn Labour members against electing the hard leftist Jeremy Corbyn as their leader, and to urge voters to shun Brexit. Neither intervention was successful.

His speech was aimed at rallying disparate and cowed pro-EU lobby groups into a coherent voice against Brexit, said the organisers of the event, the Open Britain campaign group.

“The road we’re going down is not simply ‘hard Brexit’. It is ‘Brexit at any cost’,” Blair said. “Our challenge is to expose relentlessly what this cost is ... and to build support for finding a way out from the present rush over the cliff’s edge.”

Blair said he was setting up a new institute to consider the Brexit question and other global issues.

Slideshow ( 5 images )

He stopped short of calling for a second EU referendum, saying the mechanism for voters to express any change of mind was a “second-order question”.

“YESTERDAY’S MAN”

The speech drew swift criticism from pro-Brexit campaigners.

“Blair is yesterday’s man,” said Nigel Farage, former leader of the UK Independence Party. “He seems to think we are going to change our minds. He clearly hasn’t grasped that, if that referendum was held tomorrow, the margin would be at least three times bigger.”

Foreign Secretary Boris Johnson, who campaigned for Brexit, said: “I urge the British people to rise up and turn off the TV the next time Blair comes on.”

Blair accused May and other ministers who had backed “Remain” in the referendum campaign of pledging to take Britain out of the single market for purely political reasons.

“They’re not driving this bus. They’re being driven,” he said.

Blair’s reputation among the British public remains tarnished by the Iraq war, an issue that resurfaced last year when a long-awaited inquiry was critical of his role in the decision to join the U.S.-led invasion of 2003.

Slideshow ( 5 images )

“You can like the messenger or not like the messenger,” Blair said. “I know that there will be a volley of abuse that will come my way for speaking.”

TOO MESSY

Once May formally triggers exit negotiations next month, her government argues that there will be no legal way to stop the countdown to withdrawal.

However, a court case in Dublin seeks to establish whether Britain can reverse the exit process without the permission of the other 27 EU member states.

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.

Since Britons would face a choice between a costly “hard Brexit” and “no Brexit”, he said, they might change their minds, and the other 27 member states would welcome them staying.

However, that perception may have shifted over the past few months.

Several senior diplomats have told Reuters this month that the political will to keep Britain in if London makes a U-turn seems to have dissipated.

Few governments anyway see it as likely that Britain may change tack, but those that have considered the issue believe many EU states would now find it unappealing to keep in a deeply divided Britain that is still unlikely to be a committed member of the bloc.

Moreover, while Brexit will be disruptive, a U-turn could be even more so, distracting Brussels at a time when it is dealing with other crises.

One senior diplomat from a country traditionally close to British positions in the EU said: “This bus has left. No one is happy about it, but we have moved on and the last thing anyone wants now is to reopen the whole issue. That would be just too messy.”