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In the wake of the news, her former co-star Nicola, 35, shared an emotional tribute on social media, writing that she was "absolutely devastated" to hear of Sarah's death.
"I can’t accept that this day has come," she wrote. "My heart is aching and all day everything we went through together has raced round my mind.
"Especially this last year since her diagnosis, as hard as the year has been, our new memories are strong in my heart," she added.
"There are so many things to say and at first it felt to personal to put them here and then I remembered that there are so many other people grieving her too.
"A part of me or us isn’t here anymore and it’s unthinkable and painful and utterly cruel. She would have loved your messages today.
"Electric girl, you made us. You gave it everything and still with a smile.
"A white butterfly flew past my window this morning before I knew, it must have been you".
Fellow band-mate, Nadine Coyle has also paid tribute, writing: "I am absolutely devastated!!"
"I can’t think of words that could possibly express how I feel about this girl & 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!"
In August last year Sarah announced to her fans that she had been diagnosed with breast cancer and explained the cancer has spread to other parts of her body in the following months.
She later told fans that Christmas 2020 was likely to "be [her] last".
And tragically, Sarah's mum Marie mum made the announcement on Instagram on Sunday (5th September).
“It’s with deep heartbreak that today I’m sharing the news that my beautiful daughter Sarah has sadly passed away," she wrote.
"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."
Marie went on: "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.
Sarah kept fans up to date as she went through treatment. In March she released her memoir titled Hear Me Out, where she discussed her career as part of the chart topping girl band Girls Aloud, her breast cancer diagnosis and her family.
In her book, she wrote: "Mum is in her late seventies now and she has become everything to me through this illness, especially since I've been staying with her.
"I know how hard this is on her too. I often worry about her, especially as she's gotten older. I can't bear the thought of anything happening to her.
"In a strange way it's comforting to know that I will probably leave this world before she does; at least then I won't have to go through the pain of losing her."
She added: "The truth is, I can't live without her now, she cares for me and helps me because, at times, I am too weak to help myself."
After receiving lots of support from her fans following the release of the memoir, Sarah said: “I’ve been inundated with lovely messages from my fans. I’m grateful beyond words for that.”
She first found fame in 2002 when she became one fifth of Girls Aloud - alongside Cheryl Cole, Nadine Coyle, Nicola Roberts and Kimberley Walsh - after auditioning for Popstars: The Rivals.
The band split in 2009 but were reunited for a reunion in 2012. Following her singing career, Sarah acted in roles including Bad Day, Freefall, Run for Your Wife and St. Trinian's 2: The Legend of Fritton's Gold, and won Celebrity Big Brother.
For advice, help or support, visit Breast Cancer Now.
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