Labour’s general secretary Iain McNicol has resigned after nearly seven years in the post.
The former union boss will “pursue new projects on behalf of the party and wider Labour movement”, he said in a statement released late on Friday.
HuffPost UK has been told that Unite’s Jennie Formby is being tipped by insiders as the favourite to succeed him in the job.
Another contender could be Unite’s Andrew Murray, a long-time ally of Jeremy Corbyn who took charge of the 2017 election campaign.
It is understood that former elections supremo Patrick Heneghan, who quit last year, has categorically ruled himself out of the running.
The GMB’s Lisa Johnson is a ‘compromise’ candidate whose name has been circulating in Labour and union circles since the announcement.
The officers of the party will meet to decide the process for the election of a successor in coming weeks.
A spokesman said that McNicol would work with NEC officers to deliver “a smooth and proper transition ensuring that the Labour Party remains on an election footing”.
McNicol was appointed in 2011 under Ed Miliband’s leadership and is the second longest serving general secretary after Larry Whitty.
Heavily criticised by some Corbyn supporters, he made a robust speech at the 2016 party conference when he issued a series of warnings not to pick on his staff, on previous Labour governments or on the PLP.
His departure means that there are now few pre-Corbyn era senior staffers left working at the party’s HQ.
Formby is a former political director at Unite the union but moved to a less high profile job as South East director two years ago.
McNicol, a former GMB union official, has long been criticised by Jeremy Corbyn supporters after the party took legal action to counter moves to widen the franchise for the leadership election in 2016.
A court case was brought against him when the NEC decided to bar new party members from voting in Corbyn’s second leadership ballot. The claimants won their case in the High Court, but the decision was overturned after a successful appeal overseen by McNicol.
His critics also blame him for the ‘purge’ of left-wing activists, although it was the party’s ruling NEC disputes panel that leads on disciplinary action, and on the suspension of local parties where irregularities are alleged to have taken place.
McNicol received a torrent of online abuse from some Corbyn supporters during the second leadership contest, despite his repeated insistence that he was strictly neutral.
But relations between him and John McDonnell reached a new low after the Shadow Chancellor suggested ‘party staff’ had tried to rig the leadership race by purging Corbyn supporters on spurious grounds.
Prior to the 2017 general election, McNicol was seen by ‘centrists’ as one of their key supporters at Labour HQ. When the party did better than expected, he was swiftly targeted by critics.
But allies of Corbyn have long wanted to remove him and others they see as not fully signed up to the new leadership’s direction.
Jeremy Corbyn said: “I would like to personally thank Iain McNicol for his long and dedicated service to the Labour Party as General Secretary.
“He has run our party’s organisation at a time of great change, including a near tripling of the membership, two general elections and the EU referendum.
“Iain has served the Labour Party and trade union movement in a wide range of roles throughout his career and is a credit to our movement. I am confident Iain will continue to play a major role in politics and our party, and will support our team inside and outside Parliament with his experience and skills.”
McNicol said in a statement: “It’s been an absolute honour and a privilege to serve as General Secretary of the Labour Party. I have now decided to move on to pursue new challenges in the service of the Labour Party and wider labour movement.
“I would like to thank our fantastic staff across the country for their brilliant work and support. A team who continuously go above and beyond to deliver for Labour. It has been a tumultuous seven years including; two general elections, a huge increase in our membership, Scottish and EU referendums.
“It is a hugely exciting time for the Labour Party after we achieved the biggest increase in our vote since 1945 at last year’s general election. I will continue to support Jeremy Corbyn’s leadership and will work closely with him and our whole parliamentary team to take Labour to victory at the next general election.”
Tributes to McNicol were paid on Twitter.
But his critics welcomed the departure too.
Richard Angell, director of moderate group Progress, said: “Iain McNicol has been ‘campaigner in chief’ and a true servant of the Labour movement as general secretary.
“He brings people together, reaches out across divides, has a wealth of experience as a former party organiser and union official that he put to great use in general election campaigns and has been a relentless advocate for Labour his whole life. We wish him well in what he does next.
“Being Labour general secretary is one of the most challenging jobs in politics and he has given up so much for the movement. He has always talked about how important his family is to him and we hope he is able to do more of what he so clearly loves - spending time with them.”