Brexit would be an act of "arson" on the international order, David Miliband will warn today.
In a speech in central-London, the former Labour foreign secretary will say a vote to leave the European Union would be a "tragic miscalculation".
"No nation in human peacetime history, never mind Britain, has voluntarily given up as much political power as we are being invited to throw away on June 23rd," he will say.
"All thrown away in a vote to leave. For what? For a cold, hard lesson in the demon of hubris, born of delusion that the world owes us a break. A tragic miscalculation which weakens ourselves, our friends and the international order on which we depend."
In the address which is being billed as the biggest warning to date about the foreign policy implications of Brexit, Miliband will say it would set off a "domino effect" that would "strengthen our enemies".
Since losing the 2010 Labour leadership election to his brother Ed, Miliband has been working as the president and CEO of the International Rescue Committee (
He will say today: "It is said that the world is increasingly divided between firefighters and arsonists.
"For centuries, Britain has been a firefighter. We have always sought to balance great powers and check the abuse of power. There is no shortage of fires that need to be put out around the world at the moment.
"This is no time for Britain to join the ranks of arsonists, and there should be no doubt that Brexit would be an act of arson on the international order."
He will add leaving the EU "is not a joke or a jape" but rather "a recipe for instability not order, derision not applause, a fall not pride".
In a warning to the 'Remain' campaign, Miliband will say those arguing for Britain to remain in the EU "must not cede passion or patriotism to the other side".
Labour leader Jeremy Corbyn has been criticised by some Labour MPs for not showing enough "passion" in campaigning in favour of the UK's membership.