Sarah Harding is being treated for "depression and alcohol addiction".
A Girls Aloud spokesman said the singer, 29, who is reported to have recently split up with her boyfriend DJ Tom Crane, 31, was in a treatment centre.
"Sarah Harding has entered a treatment centre for depression and alcohol addiction. There will be no further comment on this matter," he said.
Sarah, who is often seen on the London club circuit and was recently seen out with a new red hairdo, has been working on her new British film Run For Your Wife.
She announced her engagement to Tom in January after he proposed during a new year holiday.
But wedding plans were put on hold after a series of bust-ups and the pair went their separate ways in September.