The £324 listing appeared in the Births, Marriages and Deaths section of the newspaper, and read: "Mr C.E.J. Horner OBE and Geri Halliwell, The engagement is announced between Christian, son of Mr and Mrs G.M. Horner and Geraldine, daughter of the late Mr L.F. Halliwell and Mrs A Parkinson."
The announcement was printed in The Times
The couple have been friends since 2009, and publicly dating since March. It had been rumoured that they were close to tying the knot.
Geri, 42, and Christian, 40, both have children from their previous relationships. The singer is mum to eight-year-old daughter Bluebell with Sacha Gervasi.
Christian, meanwhile, recently split from his partner of 14 years Beverley Allen, with whom he has a one-year-old daughter Olivia.
Geri and Christian's newspaper notice follows that of Sherlock star Benedict, who placed a similar announcement in last week's paper revealing his engagement to theatre director Sophie Hunter.
Other celebrities to have revealed their engagements in The Times include Eddie Redmayne to Hannah Bagshawe and comedian David Mitchell to Victoria Coren.