The singer, nicknamed Posh Spice, shared a post to Instagram to her nearly 40 million followers of herself with Bunton, Geri Halliwell-Horner and Melanie Chisholm.
The 51-year-old wrote: "Happy birthday to the most beautiful soul @emmaleebunton I love you girls so much @gerihalliwellhorner @melaniecmusic."
The chart-topping girl group gathered to celebrate after Bunton, the youngest of the band and nicknamed Baby Spice, turned 50 on Wednesday.
Melanie Brown, also known as Mel B or Scary Spice, was not pictured with her bandmates but had shared a carousel of pictures on Instagram to celebrate on Bunton's birthday.
Fans flooded Lady Beckham's post with excited comments, as the photo follows speculation about a possible reunion as the 30th anniversary of the Spice Girls' hit single Wannabe approaches.
Lady Beckham's husband, former football star David Beckham, commented on the post: "This made me happy. I can only imagine how the spice girls fans feel."
He tagged the members of the girl group and added: "Special night celebrating Emma."
Bunton shared the post to her Instagram story and commented: "Love you all so much."
Other stars who commented include singer Nicole Appleton, who wrote: "Loved seeing you all!"
Chisholm, also known to fans as Mel C or Sporty Spice, shared the same photo on Instagram to her 1.2 million followers, and wrote: "The most beautiful night celebrating our @emmaleebunton over 30 years of friendship, love you all more than ever."
Halliwell-Horner, or Ginger Spice, shared the image to her page.
She tagged her bandmates, and the caption read: "Birthday celebration."
The girl group shot to fame with their debut single Wannabe in 1996 and followed it with hits including Stop and 2 Become 1.
After two years at the top of the charts, Halliwell-Horner shocked the world when she left in 1998, citing "differences between us," and in December 2000 the rest of the group went their separate ways, announcing an indefinite hiatus.
The Spice Girls reunited in 2012 for the closing ceremony of the London Summer Olympics and in 2019 for their Spice World tour - which did not feature Posh Spice.
Since then, the chart-topping girl group has been the subject of speculation about a return to the stage.
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