Jeremy Corbyn’s chief of staff Karie Murphy is now Labour’s acting General Secretary, The Mirror understands.
It comes a few weeks after Labour’s General Secretary Jennie Formby announced she would begin receiving treatment for breast cancer.
Ms Murphy first rose to prominence in 2013 when she tried to become Scottish Labour’s candidate in Falkirk after then MP Eric Joyce was expelled.
But the contest was embroiled in allegations of irregularities, leading to an investigation by Labour.
Ms Murphy was cleared of wrongdoing, alongside Unite the Union.
But the controversial episode led to the Collins Review, which changed the way Labour selects its leader – introducing the system which swept Mr Corbyn to power.
Ms Formby, who is the party’s most senior official, told staff and supporters by email and a message on social media a few weeks ago.
In the message, she paid tribute to the NHS care she has received in Southampton and said a Labour government would protect the health service and increase funding for cancer research.
In the post Ms Formby wrote: “I am writing to you with some personal news.
“I have been diagnosed with Breast Cancer and will be starting my treatment this week.
“Although this news has come as a shock to me and my family, I feel so grateful that I have access to and support from our wonderful NHS.
“I’m incredibly lucky that we have amazing teaching, research and women’s hospitals in Southampton.
“Breast Cancer has had so much investment that outcomes are really positive, but I know that other cancers have less funding for research and treatment and that access to such good cancer care isn’t the same all across the country.
“I feel pride that we are all working together for a Labour government that will change this.
“I will continue to work as much as I can. For those days when I can’t, the Executive Director and my direct team will make sure everything carries on as usual.”
