Kimberley Walsh says her "heart is broken" after Sarah Harding revealed she has been diagnosed with breast cancer.
Sarah, 38, told her fans she is having weekly chemotherapy sessions and said: "I am fighting as hard as I possibly can."
She shared a photo of herself in her hospital gown next to machines on Instagram.
Her former bandmate Kimberley said she would be with Sarah every step of the way.
She posted online: "My heart is broken. You are so strong and brave and we are with you every step of the way."
Sharing the heartbreaking news with fans, Sarah said: "I hope you are all keeping safe and well during these uncertain times. 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.
"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.
"I'm currently undergoing weekly chemotherapy sessions and I am fighting as hard as I possibly can.
"I understand this might be shocking to read on social media and that really isn't my intention.
"But last week it was mentioned online that I had been seen in hospital, so I feel now is the time to let people know what's going on and this is the best way I can think of to do so."
She added: "My amazing mum, family and close friends are helping me through this, and I want to say a thank you to the wonderful
"NHS doctors and nurses who have been and continue to be heroes.
"I am doing my very best to keep positive and will keep you updated here with how I'm getting on. In the meantime I hope you'll all understand and respect my request for privacy during this difficult time.
"Sending you all so much love....xx"
Sarah rose to fame alongside Cheryl, Nadine Coyle, Nicola Roberts and Kimberley on ITV's Popstars: The Rivals in 2002.
Her Girls Aloud bandmate Cheryl simply shared a broken heart emoji in response to the news.
