Girls Aloud were Britain’s biggest-selling girl group of the 21st Century, selling 4.3 million singles and four million albums. They scored 21 UK top 10 singles between 2002-2012, including four number ones.
Harding revealed her diagnosis last summer and said the cancer had moved to other parts of her body.
Her mother, Marie, announced her daughter’s death on Instagram, describing her “beautiful” daughter as “a bright, shining star”.
Girls Aloud bandmate Nadine Coyle said she was “absolutely devastated”.
“I can’t think of words that could possibly express how I feel about this girl and what she means to me. I know so many of you will be feeling this way,” she wrote on Instagram.
Harding was a 2002 contestant on Popstars: The Rivals, an ITV talent show that sought to assemble a new girl band and boy band. She gained the final spot in the group which became Girls Aloud alongside Nicola Roberts, Nadine Coyle, Kimberley Walsh and Cheryl Cole (then Tweedy).
Harding was born in Ascot, Berkshire, on 17 November 1981, and later moved to Stockport, Manchester.
Former Spice Girl Geri Horner, a judge on Popstars: The Rivals, tweeted: “Rest in peace, Sarah Harding. You’ll be remembered for the light and joy you brought to the world.”
Fellow judge Louis Walsh, who helped create the group, said he would remember her with “great fondness”. He added: “She was always fun and the life and soul of the party. Anytime she came to Dublin in the early days, we went out to the Pod nightclub and would be the last to leave.”
Harding was considered the group’s wild one, smashing a phone box in the video for No Good Advice and recording some of their most outrageous ad-libs.
When Girls Aloud dissolved in 2013, Harding pursued an acting career. She appeared in independent films, soap operas and reality TV shows; while still recording new music.