John Spellar, who previously called for Corbyn to resign over divisions during last year's Syria vote, said the 60,000 members of his leader's unofficial campaign team were "deplorable" for letting Government ministers get away with misrepresenting data on NHS weekend death stats.
Speaking to The Huffington Post UK, the Warley MP called Momentum members left-wing "extremists" who were making his party unelectable and leaving ordinary working people to suffer.
"What I find so deplorable about [Momentum members] is they are actually enabling this government to get away with all sorts of things.
"They're largely getting away with it. That's the real criticism; this malign government is being allowed to rig the electoral system and getting away with appalling attacks on people while all Momentum are doing is trying to change very longstanding Labour policy. It's civil war.
He added: "Many of them are Trotskyists."
Quizzed on news shadow chancellor John McDonnell has called for the unit that vets membership applications for entryists to be scrapped, Spellar said: "It's just absurd.
"We are a democratic Labour Party - and therefore people who are not democratic are not in it to support us but instead advance their own political Trotskyist polices.
"They have no place in the Labour Party."
He reasoned that Momentum were leading Labour on a road to electoral pummelling in 2020, and that voters in Northern and Midlands constituencies would be left worse-off because the party was being pushed to the political fringes.
"It’s not just the Labour Party who loses - it’s ordinary working people, especially in parts of the North and the Midlands."
Referencing an appearance earlier that day on the BBC's 'Daily Politics' programme when Spellar admitted to using "Momentum member" as an insult, he added: "That’s why I get so incensed about it and why I quite happily made those comments."
Watch the fiery exchange between him, Peter Hitchens and columnist Zoe Williams below...