Cheryl, Nadine Coyle, Kimberley Walsh and Nicola Roberts have paid tribute to their former bandmate Sarah Harding following her death on Sunday.
"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 you Sarah... farewell."
Harding, aged 39, revealed she had breast cancer in August 2020.
Earlier this year, she said the cancer had spread and doctors had told her she would not see another Christmas.
The singer "slipped away peacefully" on Sunday morning, her mother confirmed on Instagram.
Roberts also posted a tribute on social media following news of Harding's death.
"I'm absolutely devastated and I can't accept that this day has come," she wrote, while sharing photographs of them together.
"My heart is aching and all day everything we went through together has raced round my mind.
She added: "A part of me or us isn't here anymore and it's unthinkable and painful and utterly cruel.
"Electric girl, you made us."
Cheryl also extended her condolences to Girls Aloud's fans, writing: "We were like an extended family for so long and we know so many of you by name.
"I wanted to make sure you knew just how much your love and continued support meant to Sarah through her most vulnerable times. She was so grateful and you really lifted her spirits when she needed you most.
Coyle wrote on Instagram: "I am 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. For now I'm sending so much love to you."
Walsh also posted on Instagram: "Beautiful Sarah this hurts so bad. To wake up and know that you are really gone is too much to bear.
"Hearing your infectious chuckle was one of my favourite things in the world. Your fire burned so bright and you loved, lived and laughed so hard. Sending love and strength to everyone who is grieving today. My heart is broken."
In an Instagram post, Harding's mother Marie described her "beautiful" daughter as "a bright, shining star".
Posting a black and white image of Harding, she wrote: "It's with deep heartbreak that today I'm sharing the news that my beautiful daughter Sarah has sadly passed away.
"Many of you will know of Sarah's battle with cancer and that she fought so strongly from her diagnosis until her last day. She slipped away peacefully this morning.
"I'd like to thank everyone for their kind support over the past year.
"It meant the world to Sarah and it gave her great strength and comfort to know she was loved.
"I know she won't want to be remembered for her fight against this terrible disease - she was a bright, shining star and I hope that's how she can be remembered instead."
Harding shot to fame in 2002 as a contestant on Popstars: The Rivals - an ITV talent show which aimed to find both a new girl band and boy band.
She made it to the final and gained the final spot in the group which became Girls Aloud - despite being the underdog according to bookmakers - alongside Roberts, Coyle, Kimberley Walsh and Cheryl (then Cheryl Tweedy, now known simply by her forename).
Dozens of other TV and music stars have been paying tribute to the singer, who 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 Pete Waterman described Harding as "a lovely, lovely person" and "a delightful young kid", adding: "She was great - enthusiasm, full of life. This is a tragedy."
Louis Walsh, who helped create and manage the group, said he would remember her with "great fondness".
"She was always fun and the life and soul of the party," he added. "Anytime she came to Dublin in the early days, we went out to the Pod nightclub and would be the last to leave."
Another member of the ITV talent show's cast, TV presenter Davina McCall, wrote it was "so, so sad" to hear the news. "A star from the get go, hugely fun and outgoing yet also somehow fragile... love to her family and friends."
Harding's former boyfriend Calum Best, a reality TV star, spoke of the "crazy fun times and adventures" they had together, saying she would be "so very missed".
Jesus this one hits home , so damn sad, so young. I hope u rest In peace Sarah we had some crazy fun times and adventures . U will be so very missed . Sending so much love to ur mom ❤️❤️❤️ pic.twitter.com/Wt3knGOtT9— Calum Best Official (@CalumBest) September 5, 2021
How can you spot breast cancer?
The most common sign of breast cancer is a lump or thickening in the breast - but there are other symptoms too.
- Change in size or feel of the breast
- Changes in the skin of the breast, such as dimpling or redness
- Fluid leaking from the nipple, outside of pregnancy or breast feeding
- Change in position of the nipple
These symptoms can be caused by other conditions, so it is important to get any lumps or changes checked by a doctor.
Breast cancer is the most common cancer in the UK, with women over 50 more at risk than the under-40s.
But there are many other factors that can increase a person's risk, including a family history of cancer and being overweight.
Many treatments are available for breast cancer and survival is generally good if the disease is detected early.