The founder of the left wing Momentum campaign group has announced he is standing down as its leader.
In an article for the Labour List website, Jon Lansman, 62, said the time had come to hand over to a new generation of activists.
“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 said.
“That is why I have decided not to contest the forthcoming elections within Momentum and instead will hand over to a new leadership.”
Momentum grew out of the successful campaign to elect Jeremy Corbyn, a long-standing ally of Mr Lansman’s, as leader of the Labour Party in 2015.
It was credited with mobilising tens of thousands of young activists, but was distrusted by many traditional Labour members who accused it of acting as a party within a party.
The failure of Rebecca Long-Bailey, the favourite of the left to succeed Mr Corbyn, to win this year’s party leadership election was seen as a sign of its waning influence.
However, Mr Lansman insisted that it had changed the party for good.
“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: “We didn’t succeed in democratising the party sufficiently while Jeremy was leader, but we must not give up now. It’s now more important than ever that Labour becomes a grassroots member-led movement.”
Angela Rayner, deputy leader of the Labour Party, said: “There is no doubt that Jon has made a big impact in politics and is a true Socialist whom has always wanted to do his best for people, he has fought injustice and inequality and will continue to do so in the future.”