The singer reunited with the Spice Girls to perform at the London 2012 event, but had to be persuaded by Emma Bunton to go out on stage.
"I was in the taxi with Emma before we started and I nearly just got out of the car," Beckham said on Bunton's Heart FM radio show.
"I was so nervous. I felt sick and Emma was so amazing. Honestly, that's how you know a real friend. If it wasn't for her, I would not have been out there."
A source told The Mirror: "Victoria played down the incident but she basically had a full-on panic attack and was in a complete state.
"Emma was brilliant and gave her a big hug, saying, 'It's all gonna be okay, you look amazing'."
Beckham went on to thank the "amazing" Spice Girls fans, dedicating the closing ceremony performance to them.
Mel C recently told Digital Spy that she wouldn't rule out another Spice Girls reunion.
Photo Gallery - London 2012 Olympics Closing Ceremony:
Copyright: Tony Marshall/PA Wire/Press Association Images