Video report by ITV News Correspondent Martha Fairlie
The pop star revealed the diagnosis on social media on Wednesday and said the disease had spread to other parts of her body.
Writing on Instagram, the 38-year-old said: "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."
Harding, born in Ascot in Berkshire, also shared a photo of herself in hospital.
The singer added: "I’m currently undergoing weekly chemotherapy sessions and I am fighting as hard as I possibly can."
She suggested she was sharing the diagnosis now after "it was mentioned online" that she had "been seen in hospital".
She wrote: "I feel now is the right time to share what’s been going on and this is the best way I can think of to do so."
In the post she thanked her "amazing mum, family and close friends" as well as the "wonderful NHS doctors and nurses who have been and continue to be heroes".
She added: "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"
Harding formed Girls Aloud with Cheryl, Nadine Coyle, Nicola Roberts and Kimberley Walsh on ITV's talent show Popstars: The Rivals in 2002.
The group shot to fame in the noughties with a a string of twenty consecutive top ten singles in the UK - including four number one - before disbanding in 2013.
Her Girls Aloud bandmates were among those who have offered messages of support.
Cheryl posted a broken-hearted emoji on Twitter while Nadine Coyle wrote on Instagram: "I love you!!!!! You have always been able to achieve miracles when needed!! I am here for all the way & always will be!!!".
Nicola Roberts tweeted: "It goes without saying that this is blindsiding. Sarah Harding you’re so loved and supported."
Steps singer Ian H Watkins also commented on her Instagram post, writing: "Stay strong xx We’re all behind you xx".
Harding has been largely out of the public eye for the last few years, after winning Celebrity Big Brother in 2017, narrowly beating singer Amelia Lily, with Made In Chelsea’s Sam Thompson coming in third.
During her time in the reality TV house, she was romantically linked to Bachelorette star Chad Johnson.
In 2016 Harding made her stage debut in Ghost – The Musical, a retelling of the hit 1990 film Ghost, which starred Demi Moore and Patrick Swayze.
It followed a turn on the Channel 4 reality show The Jump, on which she suffered a knee injury and was forced to pull out.
What are the signs and symptoms of breast cancer and what should you do if you're worried?
UK charity Breast Cancer Care outlines the following symptoms as possible signs to look out for:
A change in breast size or shape
A lump or area that feels thicker than the rest of the breast
A change in skin texture such as puckering or dimpling (like the skin of an orange)
Redness or rash on the skin and/or around the nipple
Your nipple has become pulled in or looks different, for example changed its position or shape
Liquid that comes from the nipple without squeezing
Pain in your breast or your armpit that’s there all or almost all of the time
A swelling in your armpit or around your collarbone
Breast Cancer Care advises anyone experiencing any of these symptoms to contact their GP.
The group stressed, however, that while these symptoms may indicate breast cancer, they can also be caused by normal breast changes, or a benign (non-cancerous) breast condition - but it is still recommended to visit your GP.