The 38-year-old hadn't been active on social media for a long time before sharing the devastating news.
And now her former bandmate Kimberley says she's been left "devastated" by the shock diagnosis, but nonetheless is providing support to her pal.
Speaking to The Sun, Kimberley said: "I am completely devastated, it's been really, really tough, but speaking to her a lot helps and keeping that contact."
She added: "She knows we're?there for her every step of the way. She's a strong girl and I love?her so much."
After Sarah announced the sad news on social media, Kimberley poured out her heartache.
The pop star esteemed Sarah's strength and bravery in opening up about her cancer battle to the world.
Kimberley said she would be with her former band mate every step of the way.
She wrote: "My heart is broken. You are so strong and brave and we are with you every step of the way."
Her comment was posted beneath Sarah's original Instagram statement, which read: "Hi everyone, I hope you are all keeping safe and well during these uncertain times."
She went on: "I've not posted on here for so long, thank you to everyone who has reached out to check in on me, it really does mean a lot.
"I feel now is the right time to share what’s been going on. There's no easy way to say this and actually it doesn't even feel real writing this, but here goes."
She added: "Earlier this year I was diagnosed with breast cancer and a couple of weeks ago I received the devastating news that the cancer has advanced to other parts of my body."
The star went on to explain that she was receiving weekly chemotherapy treatment and assured fans she was fighting the disease "as hard as I possibly can."
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