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Some of Labour’s best-known MPs are under threat from a flood of radical activists in their seats, leaked internal data reveals today.

A secret document seen by the Evening Standard shows top figures including former deputy leader Harriet Harman, Labour star Chuka Umunna and ex-mayoral hopeful David Lammy are at risk.

It reveals activist numbers spiking in a string of London seats, where fears are rising that they will overrun local branches and oust moderate MPs.

One senior Labour figure told the Standard: “Many of these people have joined the party already and there is now a drive to get the others in as well. Even if there isn’t a mandatory re-selection rule brought in they will find other opportunities.”

The leak reveals there are 872 of the potential recruits in Ms Harman’s constituency. Another 659 are poised to be drawn into Mr Umunna’s constituency party. In Mr Lammy’s back yard there are 740. In the seat of former deputy leadership candidate Stella Creasy there are 725.

Rochdale Labour MP Simon Danczuk, already subject to a petition calling for his de-selection, said: “With these numbers it’s quite conceivable that some MPs will find themselves having a trigger ballot and facing a potential de-selection.”

Nerves have been frayed since the Corbyn leadership campaign was relaunched as an internal pressure group called Momentum, which is organising in some areas including Lewisham.

Its leaders deny it was set up to carry out a purge, but its website makes clear it exists to promote the Left.

The Trotskyist Worker’s Liberty Party has admitted its members helped launch a Momentum branch in Vicky Foxcroft’s Lewisham seat, with the aim of changing delegates sent to party conference next year.

The far-Left Socialist Workers Party has also urged members to join Momentum.

London’s Communist Party declared full support for Labour leader Jeremy Corbyn on welfare cuts, but failed to spell his name right on the press release.

In a document entitled “Corbin correct on tax credits”, the party challenged Labour MPs who were not supporting the leader.