Labour is on "life support" and the party needs to be saved from Jeremy Corbyn's leadership, Lord Mandelson declared on Monday evening (20 February).
The co-architect of New Labour launched the scathing attack as he spoke with Jewish Chronicle editor Stephen Pollard at the JW3 community centre in North London.
"[Labour] is not dead and it can recover, it can pass out of the accident and emergency room in due course, if and when we get a new leader," Mandelson said.
"The problem with Jeremy though is not that he is a sort of maniac – it's not as though he is a nasty person.
"It's that he literally has no idea in the 21st Century how to conduct himself as a leader of a party putting itself forward in a democratic election to become the government of our country."
The former business secretary, who held cabinet posts under Tony Blair and Gordon Brown, later admitted that he works "every single day" to "save" Labour from Corbyn.
"Why do you want to just walk away and pass the title deeds of this great party over to someone like Jeremy Corbyn? I don't want to, I resent it and I work every single day in some small way to bring forward the end of his tenure in office," Mandelson said.
"Something, however small it may be – an email, a phone call or a meeting I convene – every day I try to do something to save the Labour Party from his leadership."
The comments come after an ICM poll, of more than 2,000 people between 17 and 19 February, put the Conservatives on 44% (+2) and Labour on 26% (-1).
News of the 18-point poll lead broke just days before the Stoke-on-Trent Central and Copeland by-elections, with Labour defending both seats on 23 February.
Mandelson, meanwhile, claimed that he and Blair had a "basin of poison" poured over their heads by the Rupert Murdoch-owned press for speaking out against the government's Brexit plans ahead of the House of Lords debating the Article 50 bill, the draft legislation needed to trigger divorce talks with the EU.
"The method of Brexit that the government has chosen to pursue is one which I firmly believe will make us a poorer, smaller, less influential country in the world. That's why I reject it," he said.
A Labour source told IBTimes UK: "The idea of Jeremy Corbyn being prime minister and implementing policies that actually benefit the people terrifies the establishment, so it's no surprise Peter Mandelson has found time in his busy schedule of spending time on oligarch's yachts to attempt to undermine him."