The 38-year-old singer – who rose to fame with the girl band after winning Popstars: The Rivals in 2002 – announced on social media that she was diagnosed earlier this year and is currently undergoing weekly chemotherapy treatment.
Harding wrote: “Hi everyone, 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.”
She went on: “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.
“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”
Her announcement prompted an outpouring of support, with former bandmate Nadine Coyle writing: “I love you!!! You have always been able to achieve miracles when needed! I am her for all the way & always will be.”
Meanwhile Ian “H” Watkins, of pop group Steps, said: “Stay Strong xxx We’re all behind you xx.”
Singer Lisa Maffia tweeted: “Oh babygirl. I will be praying for you everyday. I pray for your speedy recovery. Love and light darling.”
Harding retired from public life last year, closing down her social media accounts on Instagram, Facebook and Twitter. Her official website also went offline.
The Love Machine singer became engaged to DJ Tom Crane in 2011 after four years of dating.
But their wedding was cancelled and she subsequently checked into a rehab facility in South African citing citing alcohol dependence and depression.
After Girls Aloud announced they were going their separate ways in 2013 Harding announced plans to release a solo album, which never materialised.
She had embarked on an acting career, appearing in BBC drama Freefall in 2009 with Dominic Cooper and making a guest appearance in Coronation Street in 2015.
The Run For Your Wife star took part in Channel 4 skiing reality show The Jump in 2016 but was forced to pull out mid-series after injuring her knee in a training session.
That same year she starred on the West End stage in Ghost: The Musical, but was replaced mid-tour as she was still suffering pain in her knee.
The singer and actress also competed in the 20th series of Celebrity Big Brother in 2017 and during her time in the house entered into a romance with US reality star Chad Johnson, despite being open about having a boyfriend in the outside world. Harding went on to win the reality show.
There have been reports that Harding and her Girls Aloud bandmates Nadine Coyle, Cheryl, Kimberley Walsh and Nicola Roberts were in talks to reunite for their 10th anniversary in 2022.