The singer - who shot to fame alongside Britney in The Mickey Mouse Club - voiced her support for Brit following last Wednesday's court address, in which the Toxic hitmaker likened her conservatorship to "slave labour" and said her family and those controlling her should be in jail.
Sharing a picture of herself and Britney when they were kids, she wrote: "These past few days I've been thinking about Britney and everything she is going through.
"It is unacceptable that any woman, or human, wanting to be in control of their own destiny might not be allowed to live life as they wish.
"To be silenced, ignored, bullied or denied support by those 'close' to you is the most depleting, devastating and demeaning thing imaginable. The harmful mental and emotional damage this can take on a human spirit is nothing to be taken lightly."
She continued: "Every woman must have the right to her own body, her own reproductive system, her own privacy, her own space, her own healing and her own happiness.
"While I am not behind the closed doors of this very layered & personal yet public conversation - all I can do is share from my heart on what I've heard, read and seen in the media.
"The conviction and desperation of this plea for freedom leads me to believe that this person I once knew has been living without compassion or decency from those in control.
"To a woman who has worked under conditions and pressure unimaginable to most, I promise you she deserves all of the freedom possible to live her happiest life.
My heart goes out to Britney. She deserves all the TRUE love and support in the world."
Christina and Britney both rose to fame in the late 1990s, and were frequently tipped against each other as rivals in the press.
Since Britney's court hearing, Iggy Azalea, Justin Timberlake and many other famous faces have spoken out on her conservatorship for the first time.
Britney has been locked in her conservatorship since 2008 following admission into a psychiatric facility.
Her father, Jamie, has had total stewardship over Britney's $60 million fortune, as well as having had the power to make medical decisions and business deals on her behalf. Jamie was then forced to split control with an attorney and a trust group in 2019.
In her statement on Wednesday, Britney told a very different story of her conservatorship from the smiles we see on Instagram, branding it "abusive", and expressing desires to "sue [her] family".
She told a judge she had been forced to work against her will, made to attend three counselling sessions a week and banned from doing simple things like seeing certain friends and being driven around by her boyfriend.
Perhaps most damningly, Britney talked of how her conservatorship was controlling her body, too. She told the court she had an IUD fitted against her will, so she wasn't able to have another child, despite her wishes to.
"I am not happy, I can't sleep. I'm so angry, it's insane. And I'm depressed," she said during the hearing. "I just want my life back".
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