Tony Blair has called on pro-European voters to take part in a "mission" to change the minds of leave voters.
Speaking at the Bloomberg headquarters in London, the former Prime Minister claimed the people didn't know what they were voting for in a full withdrawal from the European Union.
He said the idea of leaving being "inevitable" was wrong.
He said: "The people voted without knowledge of Brexit's true terms. As they become clear, it is their right to change their mind.
"Our mission is to persuade them to do so. I don't know if we can succeed.
"This is not the time for retreat, indifference or despair; but to defend what we believe."
Foreign Secretary Boris Johnson dismisssed Mr. Blair's speech, claiming the Government would make a success of Brexit.
He said: "I urge people, instead of rising up against Brexit, to rise up and turn off the TV whenever Tony Blair is on."