The Mirror reports that the Formula One boss millionaire proposed to Geri a few weeks ago, after a six-month courtship.
According to the paper, Geri and Christian have been telling friends of their plans, but have yet to make a public announcement because of sensitivity towards his family.
Christian split from his long-term partner last year, only six months after the birth of their daughter, and some in his family are reported to remain unsure about his relationship with Geri.
For Geri, though, it finally looks as though the romantic stars have aligned after years of misfires - including romances with Chris Evans and a fleeting fling with Russell Brand.
She has a daughter from her relationship with film director Sacha Gervasi.
Geri will be the third Spice Girl to make if official, following Victoria Beckham and Mel B.