Harriet Harman
There are 872 new party members in Harriet Harman's constituencyGetty

Leading Labour MPs could be at risk of losing their seats following the arrival of large numbers of radical activists in their constituency parties. The MPs who may be under threat include former deputy leader Harriet Harman, rising star Chuka Umunna and former minister and London mayoral candidate David Lammy.

Activist numbers are spiking in a series of London seats, raising concerns that local parties will be taken over and moderate MPs ejected.

A secret internal Labour party document uncovered by London's Evening Standard newspaper showed the extent of the flood of new members, many of whom appear to be far-left infiltrators.

An unnamed senior Labour figure said: "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 leaked document discloses that there are 872 recruits in the process of joining Harman's constituency party, 659 joining Umunna's local party, 740 are poised to enter Lammy's local branch while 725 are set to join Stella Creasy's branch.

While no attempts as yet have been made by radical activists to de-seat any of these MPs, there are fears that this could soon follow.

Rochdale's MP Simon Danczuk is already the target of a petition calling for his de-selection. He said: "With these numbers it's quite conceivable that some MPs will find themselves having a trigger ballot and facing a potential de-selection."

Anxieties have been stoked further following the relaunch of the Jeremy Corbyn leadership campaign as an internal party group called Momentum, which is already organising in some constituencies, including Lewisham in south-east London.

The Trotskyite Worker's Liberty Party said its members have helped to set up a Momentum branch in Lewisham, with the intention of changing the delegates to be sent to the Labour party conference next year.

The Socialist Workers Party, another far-left, Trotskyite party, is urging its members to join Momentum. Momentum's leaders deny it was founded in order to carry out purges of moderate MPs. However, the group's website states clearly that it will promote the Left.