The founder of the Labour grassroots campaign group Momentum, Jon Lansman, has announced he will step down as its chairman next month.
Mr Lansman, a close ally of ex-Labour leader Jeremy Corbyn, said it was time to “hand over to a new leadership.”
The left-wing group was formed out of the campaign that supported Mr Corbyn in his successful 2015 leadership bid.
Labour’s deputy leader Angela Rayner said Mr Lansman had made “a big impact on politics”.
Announcing his departure on the Labour List website, Mr Lansman said Momentum was “a mass of dedicated activists fighting for a better world” but said he would not miss “operating against a backdrop of warring factions, abuse and hatred”.
He also suggested he would remain a member of Labour’s ruling National Executive Committee (NEC), which he said was “not fit for purpose”.
He added that the group “must not give up” on democratising the Labour party which it “didn’t succeed… while Jeremy was leader”.
Mr Lansman founded Momentum in 2015 to support Mr Corbyn’s leadership bid.
In January 2018, he was elected to the NEC, calling the result a victory for “21st Century socialism”.
Later that year he joined the race to be Labour’s general secretary before dropping out of the race to focus on his role on the party’s governing body.
He argued for a much greater say for Labour members in the running of the party.
He called for an end to the era of centralised “command and control” in the Labour Party, in which the views of members were “too often ignored” and over-ruled at the party conference.
Mr Lansman, who has been a leading figure on Labour’s “hard left” for four decades, has been criticised by some within Labour who have viewed Momentum as a party within a party.
Following Labour’s crushing defeat at the last election, former Labour home secretary Alan Johnson called Momentum “a cult”.
Meanwhile, former Labour deputy leader Tom Watson blamed Mr Lansman for trying to oust him from his position last year in an internal row which threatened to overshadow the party’s conference.
Shadow education secretary Rebecca Long-Bailey wished Mr Lansman all the “very best for the future” in a tweet, saying he was “a tireless voice for Labour Party democracy for over 50 years”.