Jon Lansman, the founder of the left-wing group Momentum which campaigned in support of Jeremy Corbyn, has announced he is standing down as its leader.
The 62-year-old said the time had come for the organisation to be led by a new generation of activists in an article written for the Labour List website.
“We are now a mass of dedicated activists fighting for a better world. And in this new era, it is time that a new generation of leaders put their ideas into practice,” he wrote.
“That is why I have decided not to contest the forthcoming elections within Momentum and instead will hand over to a new leadership.”
Momentum was founded in 2015 following the election of Jeremy Corbyn as leader of the Labour Party, with the stated aim of transforming the Labour Party.
It was hailed as an effective grassroots force by supporters, while detractors criticised it for being reminiscent of Militant – the Trotskyist entryist group which aimed to push the party to the left until it was proscribed in the 1980s.
Momentum had attempted to mobilise behind Rebecca Long-Bailey during the party’s recent leadership election, but when she came a distant second to Sir Keir Starmer it was seen as a signal the party was heading in a new direction.
Mr Lansman has however insisted that Momentum had fundamentally changed what Labour stood for.
“Thanks to our movement, Labour can never again be the party of illegal invasions, anti-immigration mugs or support for Tory policies of austerity and privatisation,” he said.
“The Labour Party is now much closer to being a socialist party than simply a party with socialists in it,” he added.
Mr Lansman remains a member of the Labour Party’s national executive committee (NEC) although his article criticises it as “not fit for purpose” in holding the party leadership to account.
The report claimed that this hostility among ex-senior officials contributed to “a litany of mistakes” that hindered the effective handling of antisemitism allegations.
The leak is now subject to an internal investigation.
Following the leak Jenny Formby, another key Corbyn ally and the party’s general secretary, quit her post – ahead of the conclusion of a probe by the Equalities and Human Rights Commission into the same issue.