Bill Clinton said Jeremy Corbyn was elected Labour leader because members wanted "the maddest person in the room", according to a leaked account.
The former US president told a private fundraising event that the party "practically got a guy off the street" following the resounding defeat by the Conservatives in last year's general election.
An account of Mr Clinton's comments at the event last year - marked "For internal use only" - has been posted on the WikiLeaks website.
He told the gathering: "If you look all over the world – the British Labour Party disposed of its most (inaudible) leader, David Miliband, because they were mad at him for being part of Tony Blair's government in the Iraq War. And they moved to the left and put his brother in as leader because the British Labour movement wanted it.
"When David Cameron thumped him in the election, they reached for the interesting conclusion that they lost because they hadn't moved far enough left, and so they went and practically got a guy off the street to be leader of the British Labour Party, who I saw in the press today said that he was really a British citizen and had real British (inaudible). (Laughter).
"But what that is reflective of - the same thing happened in the Greek election - when people feel they've been shafted and they don't expect anything to happen anyway, they just want the maddest person in the room to represent them.
"And that is perfectly psychologically understandable and predictable."