Jennie Formby, a key ally of Jeremy Corbyn, has announced she is standing down as Labour’s general secretary.
Ms Formby said she had applied for the job because she wanted to support Mr Corbyn, who stood down as Labour leader after December’s general election defeat.
His replacement Sir Keir Starmer was elected last month.
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Ms Formby said it was the “right time” to step down.
Last week Labour launched an inquiry after a leaked internal report, reportedly ordered by Ms Formby, appeared to reveal the hostility of senior party officials towards Mr Corbyn.
The almost 900-page document, which looked in depth at the antisemitism crisis which engulfed the party under Mr Corbyn’s leadership, also included the names of whistleblowers and former party staff.
As well as its contents, the probe will investigate how the report was commissioned and circulated.
Ms Formby is reported to have told Labour’s ruling National Executive Council (NEC) that she has no estimate of how much the leak could cost the party if it leads to a series of lawsuits and fines.
In a statement she said she had applied for the general secretary role two years ago “because I wanted to support Jeremy Corbyn.”
She also thanked Mr Corbyn, party members and colleagues, who she said had given her support during a “very challenging period” during which she received treatment for breast cancer.
She added: “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”.
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”.
Angela Rayner, who was elected the party’s deputy leader last month, said she had “no doubt” Ms Formby would “stand squarely behind us as we continue to fight for social justice and the Labour government our country so desperately needs.”
