Sarah Harding
“Grief is such a harsh experience and an absolute rollercoaster of emotions," the singer wrote on Instagram.
“Although we knew this day would arrive I am somehow still feeling at a loss for words."
The singer and actor said Sarah's "infectious chuckle" was “one of my favourite things in the world”.
Sarah's fearlessness, shining personality and, of course, talent helped propel Girls Aloud to stardom.
The singer's death was announced on Sunday by her mother.
Geri Horner and Davina McCall – who met Sarah back in her Popstars: The Rivals days – are among those to pay their respects.
The chart-topping star revealed last year that she'd been diagnosed with breast cancer.
The Girls Aloud star says she was given "a little bit of positive news" at the end of last year.
The Girls Aloud star also told of how she spent nearly two weeks in an induced coma with sepsis.
“Thank you for the love and support, on bad days it helps me so much,” the Girls Aloud star tweeted.