Momentum calls on left to unite to prevent Labour being 'taken over by centrists' in wake of Keir Starmer victory
Momentum was set up following Jeremy Corbyn's first leadership victory.
Momentum has called on factions on the left to unite to stop Labour being "taken over once more by centrists" following Keir Starmer's leadership victory.
The pro-Jeremy Corbyn campaign group said its wing of the party was "disastrously split" and needed to come together as soon as possible.
As well as seeing their leadership candidate, Rebecca Long-Bailey, comprehensively beaten by Sir Keir, Momentum also lost the battle for three seats on Labour's ruling National Executive Committee.
In a statement, Momentum called for "unity, reflection and comradely debate on the way forward" among Labour's various left-wing factions.
The group said: "In the wake of Keir Starmer’s first round victory in the Labour leadership contest, it is clear that Momentum and the left cannot continue as it has been.
"Unlike previous years, Momentum endorsed candidates won neither the OMOV [One Member One Vote] vote for leader nor for the NEC, where the left vote was disastrously split and cost us our majority.
"At the present moment, our movement is in great peril of making the same mistakes we did after our defeat in the 1980s. We risk retreating from the challenge of making our ideas mainstream, fighting between ourselves and allowing our party to be taken over once more by centrists."
The statement added that despite "some incredible successes" in the past, the Labour left was now at risk of going backwards.
"Momentum should be the epicentre of the Socialist Labour Left," they said. "We should strive to unite that left under our banner.
"Through our democratic processes and engagement with our members, we will assess the strengths and weaknesses of the past few years and reach out to many left Labour members who feel they have been pushed away.
"It is our intention to ensure we give members the opportunity to bring forward ideas about engagement and how we move forward with a common socialist agenda that reaches out to our communities."
Momentum is owned and run by Jon Lansman, a close ally of Mr Corbyn and key figure in Ms Long-Bailey's leadership campaign.
However, he angered many on the left by attacking the rise of anti-semitism in the Labour party.