Jon Lansman, the founder of the grassroots Momentum group which helped propel Jeremy Corbyn to the Labour leadership, has announced he is pulling out of the race to be the party's next general secretary.
In a statement posted on his Twitter feed, the veteran leftwinger said that having succeeded in opening up the contest, he had decided to "step back" and concentrate on his role on the party's ruling national executive committee (NEC).
His decision earlier this month to challenge the frontrunner - the Unite union's former political director Jennie Formby - threatened to open up a bitter battle between Mr Corbyn's traditional trade union allies and those, like Mr Lansman, who wish to hand power to the mass membership.
Victory for Ms Formby, who is reported to have the support of Mr Corbyn and shadow chancellor John McDonnell, will further tighten the Labour leader's grip on the party machine.
In his statement, Mr Lansman made clear he still expected to see a contest and he urged others in the party - particularly women - to put their names forward.
"I've also had a number of members get in touch to let me know they are applying for the role of general secretary," he said.
"With two days until the deadline, I reiterate my call for Labour members, especially women, with talent and experience to consider submitting an application."
He also indicated that he would continue to campaign to replace Labour's "old top-down model" with a "modern, open and transparent, pluralist, participative democracy".
"We must draw a clear line between our renewed and reinvigorated mass-membership party and previous eras of command and control - where the views of members and affiliates alike were too often ignored, party conference overruled and the NEC disrespected," he said.
The NEC will vote later this month on a successor to the moderate Iain McNicol following his dramatic departure from the post.