In a further sign hardliners are tightening their grip on the party, Tim Dwelly quit as Labour group leader on Cornwall Council.
A party member for more than three decades, he declared it was “no longer Labour”.
Momentum activists said “Dwelly’s resignation is no bad thing” and that it showed the group was winning “the battle for the soul of Labour”.
He is the latest in a series of senior local Labour chiefs ousted by militants.
They include Haringey Council leader Claire Kober, who left after accusing sections of the party of “sexism” and “bullying”.
Momentum has also moved to seize control of “moderate” Labour councils in Manchester and Leeds.
Mr Dwelly wrote on Facebook: “I feel the Labour Party I joined all those years ago is no longer Labour. Many of those running the party in Cornwall today only joined to take it to the far-Left.
“Many of them are now actively attacking elected Labour councillors like me. I have been told bluntly that they want to get rid of me.”
His departure comes after infighting involving the chair of Truro and Falmouth Momentum, who resigned citing “lies, scaremongering and slander”.
Tory Plymouth MP Johnny Mercer said: “Across the country the hard-Left are taking over Labour, hounding out moderates.
“The hard-left who have infiltrated Labour support massive council tax hikes and disruptive bin strikes – and working people would always pay the price with higher taxes and worse services.
“Only the Conservatives are working to build a stronger, fairer economy, while Labour are only interested in playing politics, not delivering for people.”
This May, some 151 councils across England will hold elections, including all 32 London boroughs.