Following news of the 39-year-old’s death, Walsh shared photos of the two together, with the caption, "Beautiful Sarah this hurts so bad. To wake up and know that you are really gone is too much to bear.”
In memory of Harding, who was diagnosed with breast cancer last year, Walsh continued: "Your fire burned so bright and you loved, lived and laughed so hard.”
Walsh, who joined Harding, Cheryl Tweedy, Nadine Coyle and Nicola Roberts in British-Irish girl band Girls Aloud on the 2002 show Popstars: The Rivals, said her heart “is broken”.
Roberts also shared pictures of Harding and her taken during the band’s early years on Instagram and wrote, "I’m absolutely devastated and I can’t accept that this day has come. My heart is aching and all day everything we went through together has raced round my mind.”
Calling Harding the “electric girl”, Roberts added that she gave it (her battle with cancer) everything and “with a smile”.
Coyle, too, posted a smiling photograph of Harding and said she was "absolutely devastated" by the news.
Music manager Louis Walsh, who helped create Girls Aloud, in a statement said he would remember Harding with "great fondness" and that “she was always fun and the life and soul of the party”.
Harding’s mother Marie announced the pop star’s demise on Sunday, saying her daughter had “slipped away peacefully”.
Enjoy unlimited access to 70 million ad-free songs and podcasts with Amazon Music Sign up now for a 30-day free trialSign up
“Many of you will know of Sarah’s battle with cancer and that she fought so strongly from her diagnosis until her last day.”
Earlier this year, Harding shared that a doctor had told her she probably wouldn’t make it till next Christmas.
In her memoir Hear Me Out, she wrote that she was "trying to live and enjoy every second of my life, however long it might be” and “having a glass of wine or two during all this” to relax.
Harding thanked her fans for the support she received after publicly revealing her diagnosis. "I’ve been inundated with lovely messages from my fans,” she said. “I’m grateful beyond words for that."
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies