Sarah's mum Marie today confirmed the news on Sarah's Instagram page, alongside a photo of the star.
She had been diagnosed with advanced breast cancer in August last year, and was due to celebrate her 40th birthday in November.
Sarah underwent a mastectomy and chemotherapy following her diagnosis in a bid to prolong her life.
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Marie said: "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.- Marie x."
Sarah was born in Berkshire and won Celebrity Big Brother in 2017, after she shot to fame as a member of Popstars: The Rivals girl band Girls Aloud.
She shared her tragic diagnosis with fans on social media last year.
In her best-selling book Hear Me Out, in which she documented her battles with breast cancer, she said: "I don’t want an exact prognosis. I don’t know why anyone would want that. Comfort and being as pain-free as possible is what’s important to me now.
“Silly little things make me happy: my lie-ins, watching Family Guy on TV through the night when I can’t sleep, roasting a chicken for Mum and me on a Sunday, if I’m feeling up to it.”
Sarah released a previously unheard song, Wear It Like Crown, in March this year - with all proceeds from the sales going to The Christie NHS Foundation where she received her cancer treatment.