Jennie Formby has stood down as Labour's general secretary after two years in the job, following the departure of Jeremy Corbyn as party leader.
Ms Formby said her two years in the job had been “a very challenging period” and "now we have a new leadership team it is the right time to step down".
She added she had agreed the "dates of this transition" with the new leaders when they were elected at the start of April.
In an email to Labour party staff, Ms Formby said a new general secretary would be "in place very soon".
The 60-year-old said she applied for the role of general secretary in 2018 to "support Jeremy Corbyn" but during her tenure she had to contend with anti-Semitism claims against the party, rows over Mr Corbyn’s leadership and she also underwent treatment for cancer.
Ms Formby added it was a "huge privilege" to be general secretary of the "largest political party in Europe" and wished the "very best of luck" to new leader Sir Keir Starmer and his deputy Angela Rayner.
“I would like to thank Jeremy, our members and my staff colleagues who have given me so much support during what has been a very challenging period, in particular when I was suffering from ill health," she said.
“I wish Keir and Angela the very best of luck in taking the party forward and leading Labour to victory at the next general election”.
Mr Corbyn thanked his "great friend and comrade" for her "commitment and hard work".
He added: "I look forward to campaigning together for the cause of Labour and social justice in the years ahead."
Sir Keir thanked Ms Formby for "her service, and for the personal and professional efforts she has made in advancing the cause she has fought all her life for".
He added: "Jennie has led our party’s organisation with commitment and energy through a period of political upheaval, including a snap general election last year.
"I wish her the very best for the future."
Ms Formby said until a new general secretary takes over, her responsibilities will be delegated to senior party staff who will work closely with the party chair to "ensure the smooth running of the party".
Labour MP Margaret Hodge tweeted that Ms Formby’s resignation was “another opportunity to draw a line under the past four years”.
“We can now better rebuild public trust in Labour, professionalise our party machine & finally eradicate antisemitism from within our ranks,” she added.