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Momentum, the grassroots group set up by backers of Jeremy Corbyn, is to curb the influence of far-left groups in it.

It is understood Momentum is to bar non-Labour Party members from taking part in some of its meetings.

The move follows mounting criticism of Momentum from senior Labour figures who have accused it of planning a purge of Labour moderates.

Deputy Labour leader Tom Watson has called Momentum "a bit of a rabble" and has warned of an "entryist problem".

Under new rules, Momentum supporters who are not Labour Party members will not be allowed to vote or take part in meetings about the Labour Party.

The move is designed to restrict the influence of organisations like the Communist Party, Left Unity, the Socialist Workers Party, the Socialist Party and the Trade Unionist and Socialist Coalition.

The new rules are due to be finalised shortly, BBC assistant political editor Norman Smith said.

The rules will still allow members of the groups - generally to the left of the Labour Party - to attend Momentum meetings on non-Labour Party issues, such as campaigning on Syria.

A Momentum source said: "This is to stop the Socialist Party doing stupid things but we're trying to be as open and pragmatic as possible."

Momentum has strongly divided opinion within the Labour Party - shadow chancellor John McDonnell has spoken at a number of their meetings.

Intimidation claims over Syria vote

Momentum says it was established to give a continued voice to the thousands of people who helped elect Jeremy Corbyn as Labour leader after becoming registered supporters of the party, and to ensure Mr Corbyn's anti-austerity policy platform was maintained.

Image copyright PA Image caption Tom Watson has called Momentum 'a bit of a rabble'

But critics have warned it risks becoming a "party within the party" and that some of its supporters are not members of Labour and do not share its values.

As well as calling it "ineffective" and "a bit of a rabble" last week, Labour's deputy leader Tom Watson said on Twitter on Monday "they have an entryist problem in a number of groups".

Caroline Flint, a former Labour cabinet minister, claimed in an interview with The Independent, that Momentum could "destroy" the Labour Party.

"They seem to be focused on interfering in the organisation of the Labour Party. That is exactly what far-left groups like Militant did in the 1980s. They act as a separate party operating within the Labour Party but with no real loyalty to the party."

But Ken Livingstone, who is a member of Momentum, and who is co-chair of Labour's defence policy review, said the group did vital campaigning work and was not a "nasty bit of work like the old Militant".

Momentum has come under fresh scrutiny in recent days amid claims MPs who voted to bomb so-called Islamic State in Syria have been subject to online intimidation - or threats to oust them as Labour candidates.

Members of the Socialist Party, a successor group to Militant, reportedly handed out a petition calling for the de-selection of Streatham MP Chuka Ummuna at a Momentum meeting in Lambeth last week and the party has led calls for Walthamstow MP Stella Creasy to be ousted by her local Labour Party.

Left Unity, which considered dissolving itself and joining the Labour Party after Mr Corbyn's election but opted instead to get involved with Momentum to support his leadership, "named and shamed" the 66 Labour "warmongers" who voted for bombing in Syria, calling for them to be de-selected.

Momentum has defended its right to lobby MPs but insisted it "strongly disapproves" of anyone engaging in threatening behaviour or de-selection threats.