The singer, who rose to fame on TV talent show Popstars: The Rivals in 2002, was diagnosed with advanced-stage breast cancer in August 2020.
In a post on social media, Sarah's mother announced the sad news of her passing today (5 September).
"It is with deep heartbreak that today I'm sharing the news that my beautiful daughter Sarah has sadly passed away," mum Marie wrote on Instagram. "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."
Marie continued: "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."
In Popstars: The Rivals Sarah became a member of girl group Girls Aloud who went on to have huge success throughout the 00s with twenty consecutive top ten hits and six top ten albums.
Following the group's hiatus in 2013, Sarah took part in a series of solo projects on screen including BBC drama Freefall and a brief stint on ITV soap Coronation Street.
In 2017, Sarah took part in Celebrity Big Brother, winning the series and a £50,000 cash prize for a charity of her choice.
Earlier this year, Sarah released an autobiography, Hear Me Out.
She heartbreakingly wrote: "In December my doctor told me that the upcoming Christmas would probably be my last."
Sarah wrote of talking publicly about her illness: "Maybe if I spoke out, as a public figure, a celebrity, it could help get the message across how important it is to get checked out if you have concerns.
"As scary as it was to go public about my diagnosis, it was the right thing to do, and the amount of support I've received is incredible.
"I've been inundated with lovely messages from my fans. I'm grateful beyond words for that."
For information on breast cancer including signs and symptoms to look out for, visit Breast Cancer Now.