Former British Prime Minister Tony Blair urged people to "rise up" and change their minds on Brexit, arguing that the vote to leave the European Union poses a threat to the country.

In his first major political intervention since the vote last June, Blair issued a rallying cry to opponents of Brexit, saying there was little clarity over what the vote meant when the referendum took place and that the government was set on "Brexit at any cost".

"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 to 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."

Current Prime Minister Theresa May has vowed to trigger Article 50 and start the process of leaving the European Union next month, and has said she envisions a clean break from the bloc, including leaving the single market. She has also warned politicians against disrupting the process.

Blair criticised May and other members of the government, who had backed "remain" in the referendum campaign, for pledging to take Britain out of the single market for political reasons even after they had offered a staunch defence of its merits.

"They're not driving this bus. They're being driven," he said.

Blair, a three-time election winner for the opposition Labour party, remains a loud voice in British politics and has sought to make targeted interventions in recent years: first to warn voters against electing Labour hard-left leader Jeremy Corbyn and later to try and dissuade voters from backing Brexit. Neither intervention was successful.

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

While he retains the support of Labour Party moderates, he is reviled by many members who feel he betrayed the party's socialist roots, and his old party has shifted far away from the pro-business centre ground he once championed.

Blair said that among the risks of Brexit, the issue of the break-up of the UK was now "back on the table" and the circumstances for nationalists were now "much more credible" than they were three years ago.

Scotland voted by 55-45 percent to remain part of the UK in September 2014, but the pro-European Scottish National Party has argued that the Brexit decision reopens the independence question, as most Scots voted to stay in the EU.