Labour's General Secretary Jennie Formby has announced she will begin receiving treatment for breast cancer.
Ms Formby, who is the party's most senior official, told staff and supporters by email and a message on social media.
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."
A Labour party spokesperson said: Jennie will continue to work as much as possible but is taking a realistic approach to her treatment.
"The party is fully supporting Jennie through this period."
