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Jeremy Corbyn’s Momentum group is poised to take over its first council after ousting sitting councillors and replacing them with Left-wingers.

A purge of moderates in Haringey means the Left would have a controlling majority after next May’s borough elections, and council leader Claire Kober, who has led the flagship borough for nine years, faces an almost certain leadership challenge.

Haringey councillor Barbara Blake, who decided to step down after being targeted, blamed Momentum for “ruthless attacks” on those they did not support.

Departing cabinet member Ali Demerci said on Twitter: “You have to be really naive to believe this is not a purge by Momentum.”

The breakthrough by Left-wingers comes amid efforts by Momentum to unseat moderates across London, including in Lewisham, Newham, Tower Hamlets and Ealing.

It follows three weeks of selection contests in Haringey in which nine sitting councillors were ousted and others quit, with new candidates chosen to replace them for the May 3 polls.

It could mean Ms Kober leads Labour into the elections on a moderate manifesto, only to face an immediate leadership challenge from one of the newly elected Left-wingers.

The Left would then be in charge of the borough, which serves nearly 300,000 people and has a budget of £693 million. Some supporters think Ms Kober would be able to rely on just 10 loyal supporters in the 49-strong Labour group after the elections.

She has been undermined by a revolt against her flagship housing policy, a public-private partnership called the Haringey Development Vehicle (HDV).

The current Labour group backed the plans by 29 to 20. But an analysis of the incoming councillors shows that will change to an overwhelming 33 to nine against. There are just a handful of selections to go.

A Left-wing source said: “She has lost her majority as a leader, and on her policy to introduce HDV. Even if she was to drop HDV, there’s still a lot of resentment on the Left towards her.” Ms Blake, who decided to step down from St Ann’s ward, said Momentum were “openly canvassing” and councillors felt “intimidated”.

Tim Gallagher, who withdrew from standing in Stroud Green, said the Left “organised so ruthlessly to get rid of us” and treated councillors as “zombie Blairites”. Cabinet member for housing Alan Strickland spoke of a “sectarian atmosphere” caused by “factional activists” before he withdrew.

Ms Kober is also chairwoman of London Councils. But Mr Corbyn fuelled opposition to her housing blueprint in his speech to the Labour conference which said regeneration too often meant “forced gentrification and social cleansing”.

The HDV proposal transfers thousands of social homes to a company owned 50/50 by Haringey and private firm Lendlease, in a £2 billion deal set to last 15 to 20 years.

A Left-wing source said Haringey council would in future be “a Corbyn-supporting council but that there is no such thing as a Momentum council”.

A source in Momentum denied masterminding a coup and said changes in Haringey were driven by anger against the HDV.

“This is shown in the selection of councillors such as Vincent Carroll, who signed a letter in 2016 calling for Corbyn to resign but was selected by local members because of his anti-regeneration stance,” they said.