Unison: Union boss Dave Prentis stepping down after 20 years
The general secretary of the country's largest union will step down at the end of the year, when his term expires.
Dave Prentis has led Unison, which represents 1.3m workers, since 2001.
In a statement, he said it had "been the honour and privilege of my life to be able to represent our incredible public service workers from across our four nations".
The union's executive committee will announce a timetable for choosing his successor by the end of July.
Unison represents staff who provide public services, working in the public, private or voluntary sectors. Mr Prentis has worked for the union for 47 years.
Sir Keir Starmer, the leader of the Labour Party - which the union is affiliated with - tweeted his thanks to Mr Prentis,
Announcing his decision to step down, Mr Prentis said: "I have always been driven by a belief in fairness and justice for all our members, especially in the past five months when they and our public services have risen to the many challenges posed by the pandemic.
"As the health crisis turns to an economic crisis, I will be here to continue to lead Unison until the end of this year.
"There's much still to do - holding the government to account for its handling of the pandemic, ensuring proper funding for our public services and a decent pay rise for all their hardworking employees."
Cabinet Secretary Michael Gove said Mr Prentis had been an "exemplary trade union leader." He said the pandemic had reminded everyone "how much we depend on the public sector workers who he speaks up for."
"I would like to extend my best wishes to him on his retirement," Mr Gove added.
Some of former Labour leader Jeremy Corbyn's most loyal supporters were from the trade unions.
Dave Prentis, however, was seen by many as a more moderate influence in Mr Corbyn's party, who would sometimes try to force the leadership into a position it didn't want to take.
For example, under Mr Prentis, Unison was one of the most prominent voices trying to persuade Mr Corbyn to endorse a remain position last year. That was backed by many in Labour's centre - and an example of Mr Prentis being happy to put pressure on the leadership.
He and Unison backed Keir Starmer for the Labour leadership. And since Sir Keir's victory, Labour's 'moderates' have been in the ascendancy.
The contest to replace Mr Prentis could influence Labour's direction.