The former Girls Aloud singer, 34, told fans she's "gutted" that an injury means she can no longer take part in the C4 series.
After reports surfaced that Sarah had quit, she took to her official Twitter account to set the record straight about the reason for her premature withdrawal.
In a series of passionate tweets to her 200,000 followers, she explained: "#TheJump sorry to say that I DID NOT QUIT but had no choice due to a ruptured ligament in my one from jumping guys [...]
"Am actually #gutted to Rcv this news today after 6 wks of slogging my ass away trimming daily for 6 wks and to make it this far surpassed even my own expectations, but all I can say is, I did the best to my ability and despite the ups and downs (literally!)."
The former Coronation Street actress went on to say: "I am so proud of how much I progressed be how far I came. Have me some incredible friends and me some amazing memories on this once in a lifetime journey, so for that and so much more, I could not be more thankful. Peace."
She added that more would be revealed on Sunday night's show (28 February).
Fans were quick to send supportive messages to the star, with one saying: "So so proud of you, you've done incredible and we've loved watching you on the show!"
Another said: "Shall miss you, you was my winner xxx Get better soon xxx."
#TheJump a lifetime journey, so for that and so much more, I could not be more thankful. Peace.— Sarah Harding (@SarahNHarding) February 27, 2016
A spokesperson for Sarah told Digital Spy: "Sarah Harding has withdrawn from the competition on medical advice after pulling a ligament in her knee during training yesterday.
"She has been advised by medics to rest and will undergo physiotherapy. Sarah has been a great competitor and we wish her a speedy recovery."
Sarah is the sixth celebrity who has pulled out on medical grounds, following Rebecca Adlington, Tina Hobley, Beth Tweddle, Mark-Francis Vandelli and Linford Christie.
Ben Cohen recently revealed that he had to have 20 stitches in his mouth when a training session went wrong.
Meanwhile, Louisa Lytton, James 'Arg' Argent and Joe Swash - who broke a bone in his shoulder during training - have all been eliminated.
Sunday's semi-final will see the remaining seven celebrities - Ben, Brian McFadden, Dean Cain, Heather Mills, Sid Owen, Tamara Beckwith and Tom Parker - take on the intense ski cross event.
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