"Although we knew this day would arrive I am somehow still feeling at a loss for words that our stunning, unique, crazy, quirky, kind and soft-hearted girl has departed," wrote Cheryl in an Instagram post.
"I love Sarah... Farewell!
Harding, 39, was diagnosed with breast cancer in August 2020.
She said that her cancer had advanced and doctors had advised her not to see another Christmas.
Her mother confirmed that the singer "slipped away peacefully" Sunday morning via Instagram.
Roberts also shared a tribute to Harding on social media after the news broke about Harding's passing.
"I'm absolutely devastated and I can't accept that this day has come," she wrote, while sharing photographs of them together.
"My heart hurts and I am reeling from the memories of our time together.
"Electric girl, we are yours!"
Cheryl also expressed her condolences for Girls Aloud's fans by writing: "We were like a extended family for so many years and we know so much of you by name.
"I wanted you to know how much Sarah appreciated your support and love during her most difficult times. She was so thankful and she really appreciated your support when she needed it most.
Coyle wrote on Instagram: "I am absolutely devastated. It's hard to express my feelings about her and the things she means to me.
"I know that many of you will feel this way. "For now, I send you my deepest love."
Walsh also posted on Instagram: "Beautiful Sarah this hurts so bad. It is hard to wake up one morning and realize that you are gone.
"Hearing your infectious laughter was one of the best things I have ever done. Your fire was so bright, and you loved, laughed and lived so well. I send strength and love to all who are grieving. My heart is broken."
Marie Harding, Harding's mom, wrote an Instagram post describing her beautiful daughter as "a bright shining star".