Ginger Spice marked the occasion with a snap of them both backstage at the girl group’s 2007 tour.
Beckham struck a model pose in a bold hot pink outfit alongside Horner, who had her hands full holding her daughter Bluebell and her other arm around a baby-faced Brooklyn Beckham.
The sweet snap was captioned: “Happy Birthday @VictoriaBeckham. This was us on tour in 2007!”
The Spice Girls account then adorably commented: “Always a Spice Girl.”
Last year Beckham confirmed that she would not be joining the band’s reunion tour as she told how her sole focus is on “empowering women through fashion”.
An unimpressed Mel B, real name Melanie Brown, has since hit out at Posh Spice, reportedly branding her a “bit of a b****” for refusing to join her, Horner, Mel C and Emma Bunton on their upcoming summer tour.
When quizzed on what she thinks of Beckham on Piers Morgan’s Life Stories, Brown supposedly said she was “gutted” she would not be hitting the road with them.
According to The Sun, Scary Spice said: “I am sad she is not going to be there. But she is busy with what she is doing with her fashion line and her family. But I am gutted that she is not going to be with us. But I am still going to hold that candle and hope that she will.”
The comments are believed to have come from the same interview in which Brown claimed she slept with Horner back in the group’s heyday.
However, the controversial revelation was declared “untrue” by Horner shortly after.
The disagreement sparked fears among fans that the tour may not go ahead.
But Bunton later confirmed the group will be hitting the road and are enjoying “peaceful” rehearsals, which include FaceTime calls with Brown, who is currently in Los Angeles.
Depsite the distance Brown confirmed she is preparing, after posting a picture of herself wearing a jet-lag mask which stops people from contracting in-flight infections.
Her caption read: “Getting vocal ready with my @humidiflyer this AM yippeee #spicetour #vocals #rehearsal getting it done.”
The group will headline three shows at Wembley Stadium in June after kicking off the tour in Dublin on May 24.