David Miliband, seen by his supporters as UK Labour's lost leader, has claimed the party under Jeremy Corbyn has not been as far from power since the 1930s.
The charity boss said Mr Corbyn had made Labour unelectable and betrayed working people with his half-hearted campaigning in the EU referendum.
Mr Miliband, beaten by his brother Ed in the 2010 leadership election, made his attack hours after voting closed in the latest contest.
Writing in the New Statesman, Mr Miliband said: "The main charge against Jeremy Corbyn is not just that his strategy is undesirable because it makes the party unelectable.
"That is only half the story.
"The real issue is that his strategy makes the party unelectable because it is in many aspects undesirable."
On Mr Corbyn's foreign policy, he said: "The half-hearted message about Europe is a betrayal of millions of working people.
"The equivocation on NATO in the face of Russia's intimidation of nations in her former sphere of influence is dangerous and throws away progressive values."
He added: "But the electorate can see through the domestic policy, too.
"Nationalisation cannot be the answer to everything; anti-austerity speeches cannot explain everything; corporate taxation cannot pay for everything.
"It doesn't add up. It wouldn't work. People are not stupid."
Also undesirable and disastrous, according to Mr Miliband, is the critique that everyone who disagrees with Mr Corbyn is in fact a closet Tory - or "Tory lite".
He said this creates a new version of history that there is no difference between Labour and Tory governments.
"This is the sectarianism that leads to the dead end of permanent opposition," he wrote.
'Wipe the slate clean'
Coinciding with Mr Miliband's onslaught on his foreign policy, Mr Corbyn has called for the appointment of a "minister for peace" and a "minister for disarmament".
In a documentary by the left-wing film director Ken Loach, he said: "There has to be a change in attitude on foreign policy.
"Let's have a minister for disarmament and a minister for peace as well who is pursuing those things around the world and looking to enforce the non-proliferation treaty rather than pretending it's an obstacle to rearmament."
Earlier, Mr Corbyn vowed to "wipe the slate clean" if he is re-elected and called for an end to "sniping and personal attacks" which he described as "destructive self-indulgence".
In his statement after voting ended, he said: "As far as I am concerned, the slate will be wiped clean this weekend.
"If I am re-elected leader, I will reach out to and work with all Labour MPs to form a broad and effective opposition to this divisive and floundering Tory government.
"And I will work to create a strong leadership team for our party, inside and outside Parliament, based on respect for each other and for all those who rely on Labour to defend their interests."
And he warned his critics: "We owe it to the millions of people Labour exists to represent to end the sniping and personal attacks, and work together for all those who depend on the election of a Labour government.
"Anything else would be destructive self-indulgence."
