The troubled singer was admitted to rehab in 2011 to address a dependency on alcohol and sleeping pills during a break from the pop group, and she was plagued with crippling nerves when she rejoined her co-stars to confirm their return last year (12).
Harding appeared with the rest of the group at a press conference in London to announce their final tour and greatest hits album, and bandmate Kimberley Walsh has now revealed her friend fell to pieces at the event.
Walsh alleges Harding started drinking to calm her nerves on the morning of the press conference, and was a tearful wreck by the time they took to the stage.
In her new autobiography, A Whole Lot of History, she writes, "On the day we did the press conference... I was really worried about Sarah. She was terrified about going out in front of the press and Tv cameras again, especially as there was so much speculation about whether or not she was getting herself back together after her time in rehab. She'd had a couple of drinks to calm her nerves, which didn't help... She was really in a bad way... Just before we were due to go on stage... She started to cry... Suddenly Sarah was freaking out. 'I can't do this,' she said, 'I can't'... (But) she pulled herself together just in time."
Walsh goes on to admit she struggled with the "moral dilemma" of scheduling a Girls Aloud tour when her friend was so fragile, revealing she feared Harding would not be able to cope with the pressure.
She adds, "I even wondered whether a tour was the right thing for her to be doing - it became a moral dilemma. I wanted the tour to be amazing, but not at Sarah's expense."
The group split after completing the Ten tour.