The 38-year-old hadn’t been active on social media for a long time before sharing the devastating news.
Sharing a photo of herself in the hospital, she wrote: “Hi everyone, I hope you are all safe and healthy during these uncertain times. Haven’t posted here for so long, thanks to anyone who contacted to check on me, it really means a lot. I think now is the time to share what is going on.
“There’s no easy way to say this and in fact it doesn’t even feel real to write this, but there you go. Earlier this year I was diagnosed with breast cancer and a few weeks ago I received the terrible news that cancer has grown to other parts of my body.
“I’m currently on weekly chemotherapy and I’m fighting as hard as I can. I understand this can be shocking to read on social media and it really isn’t my intention.
“But last week it was mentioned online that I had been seen in the hospital so I think now is the time to let people know what’s going on and this is the best way to do it. .
“My mom, family and close friends are great helping me through this, and I want to thank the wonderful NHS doctors and nurses who have been and continue to be heroes. I’m doing my best to stay positive and I will. Keep up to date here with how I’m going.
“In the meantime, I hope you all understand and respect my request for confidentiality during this difficult time. Send you all so much love… .xx ”
Following her tweet, former bandmate Cheryl broke her months-long silence on Twitter to share a heartbroken emoji.
Her compatriot Nadine Coyle posted: “I love you !!!! You have always been able to perform miracles when needed !! I am there all the way and always will be !!! ”
In March, it was reported that Sarah had gone into hiding and was retiring from the limelight after her years of glory in the group, as an actress and after her much-discussed appearance on Celebrity Big Brother.
Sarah was last seen in public at the National Film Awards in London in March 2018.
In addition to avoiding private events, Sarah also stopped posting to her social media accounts.
Prior to today, she last shared a photo on Instagram in May 2018.
Its official website has also been taken offline.
Sarah rose to fame as the fifth girl aloud when she was trained on the ITV Popstars: The Rivals show in 2002.
They quickly became five of the world’s most famous women, racking up the top ten singles and albums, before taking a break.
The group reunited in 2002, but broke up the following year.
Sarah has also landed acting roles during her career.