Cheryl Cole, Kimberley Walsh, Nicola Roberts and Nadine Coyle told Harding that she had to seek help for her ongoing depression and alcohol issues if she wanted to be part of the band's comeback.
Harding entered rehab last week following news of her relationship "break" from fiancé DJ Tom Crane.
"It's been one of the worst-kept showbiz secrets that the girls are going to be reforming next year to mark their 10-year anniversary," a source told the Sunday Mirror. "They want these to be their best shows yet to make sure they come back with a bang and are coming up with some huge ideas.
"But as preparations started to get under way, the girls realised that, because of her party lifestyle, Sarah was unlikely to be in a fit shape to do it. Sarah was so excited about getting back on stage with the girls after they took a break in 2009, and sometimes you just need to hear the truth from your closest friends.
"So Sarah decided to check herself into a rehab facility in America of her own accord to prove to the girls that she was serious about the tour - and also to try to win Tommy back. The girls are fully supporting her and are saying they will be with her every step of the way."
Girls Aloud are widely expected to reform for their tenth anniversary next year, with rumours suggesting they will play a complete a three-week tour of the UK.
Watch Girls Aloud performing 'Something Kinda Ooooh' live at V Festival 2008 below: