The actor has invited her to accompany him on his UN mission to Darfur when the band's reunion tour ends.
He explained: “She was always my favourite Spice Girl. I loved her feisty approach and the fact she wasn’t afraid to have an opinion.
"Her work with Unicef and [sexual health charity] Marie Stopes in the third world was inspiring. And her address at a UN Youth summit in 2000 was brave - it's so easy to not get involved and she just gritted her teeth and went for it.”
A source told The Daily Star: “George really admires Geri’s tenacious attitude.”
Halliwell recently claimed that it is unlikely that the Spice Girls will make a profit from their tour.