Mel B and Victoria Beckham congratulated her on Twitter, writing:
Earlier on in the day Halliwell posted clues on Twitter and Instagram, in the form of a series of pictures of something old, something new, something borrowed and something blue.
Halliwell and Horner have been friends since 2009.
They have been publicly dating since March 2014, and got engaged in November.
They announced their happy news in the births, deaths and marriages page in The Times.
Their announcement 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."
A few days after their official announcement, Halliwell was photographed out and about showing off an impressive diamond ring.
Both have children from previous relationships. Halliwell's daughter, Bluebell, is now nine and Horner has a baby daughter, Olivia, from his previous partner of 14 years, Beverley.
Halliwell's former love interests include Russell Brand, Henry Beckwith and David Walliams.
In 2005, while she was dating Sacha Gervasi, a screenwriter, she fell pregnant with her daughter.
Prior to that she had been linked to Robbie Williams, Chris Evans and polo player Jamie Morrison.
Before she started dating Horner, she was dating millionaire Anton Kaszubowski.
- AAP
