The fivesome was originally formed on ITV talent show Popstars: The Rivals back in 2002.
Following the formation, the BRIT Award-winning girl band scored 20 consecutive top 10 singles in the UK, including four number ones, with hits such as Sound Of The Underground and I’ll Stand By You.
After selling more than 4.3 million singles and four million albums in the UK alone, the group officially disbanded in 2013.
Celebrating all that they achieved, the 41-year-old shared a slew of throwback photos featuring her bandmates Harding, Cheryl Tweedy, Nadine Coyle, and Nicola Roberts, throughout their years together.
Writing alongside the post, she penned: “Happy 20 years girls! Wow I am proud of what we achieved and love you all much.
“There is a huge hole in my heart that Sarah isn’t here to celebrate this with us but I am looking through pictures, sobbing and remembering all the amazing times we shared.
“A girl band is a bond that no one can ever break. The memories will always stay with us forever #GA20 #20years.”
The band went their separate ways in March 2013 but have reunited several times in the past year in honour of their late friend Harding, who died last September aged 39 from breast cancer.
In October this year, the girls came together to host a fundraising gala, titled The Primerose Ball, for cancer research at The Londoner hotel.