In the image, taken at an album launch in 2001, Spears can be seen laughing alongside Timberlake and Portman, who appear to be sharing a joke with the young Jamie-Lynn. Spears’s sister would have been around nine or 10 years old at the time.
Spears published the image to her Instagram, while dedicating it to her little sister, who turned 30 on 4 April.
“Happy belated birthday to this beautiful soul,” Spears wrote in a caption. “I love you so much and damn it I wish I was as smart as you at 30!!!! It’s weird to say that I was basically your momma when you were younger and yes I’m still older but your soul is and always has been wise!!!!”
Spears added that she was “truly and genuinely inspired” by her little sister’s “path”, writing: “I’m so f***ing proud to call someone as beautiful inside as you my sister!!!!”
The unexpected gathering of people in the photograph led to confusion and shock amid Spears’s fans.
“This is confusing,” wrote one person. “What a strange picture to choose,” added someone else.
Another fan added: “This is so random but so iconic.”
Unusually, the photograph appeared to have been significantly airbrushed, with Spears or someone on her social media team digitally blurring a watermark visible when it was originally shared on Instagram by the throwback account @noughtiez. The airbrushing was so extensive, however, that half of Spears’s sister’s head appeared to have been removed.
While Spears and Timberlake were a superstar couple between the years of 1998 and 2002, Spears’s one-time friendship with Portman is less well known.
The pair were both understudies in the same Broadway show when they were children, and later reconnected once both had become famous. Spears has repeatedly said that she would want Portman to play her in a film inspired by her life, and last month named the Oscar winner as one of the “cool ladies” who had most inspired her.
Other women named as Spears’s biggest inspirations included Sharon Stone and Sarah Jessica Parker.
Spears, who has been at the centre of a legal battle with her father in recent months, broke her silence in March on a high-profile documentary exploring the court-ordered conservatorship she has been placed under since 2008.