Iain McNicol, Labour’s second longest serving general secretary, has resigned after seven years in the post.
McNicol will pursue new projects on behalf of the party and wider Labour movement and is expected to be replaced by an ally of leader Jeremy Corbyn.
The former GMB boss said: ‘It’s been an absolute honour and a privilege to serve as General Secretary of the Labour Party. I would like to thank our fantastic staff across the country for their brilliant work and support.
‘It has been a tumultuous seven years including; two general elections, a huge increase in our membership, Scottish and EU referendums.’
He added: ‘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.’
Despite heavy criticism from supporters of Jeremy Corbyn the Labour leader tonight paid tribute to McNicol.
He 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.
He added: ‘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.
HuffPost UK reported Unite’s Jennie Formby is being tipped by insiders as the favourite to succeed McNicol, who was appointed by Ed Miliband in 2011.
The general secretary is a position which is appointed by the party and is not an elected role.