The once scantily clad pop star will be an ambassador for the UN population fund to promote awareness of reproduction health issues in Britain.
She will also work with the Marie Stopes charity, which runs abortion clinics and advises on contraception.
Ms Halliwell, 26, who as Ginger Spice led the group to the top of the pop charts and then promptly walked out during their world tour in May, has already proved her worth to charities in Britain. With the other four Spices, she publicised last year's Remembrance Day poppy appeal. Ms Halliwell, who suffered a breast cancer scare when she was 18, also launched Breast Cancer Awareness Month this month, visiting inmates at Holloway Prison and sixth-formers at a London school to stress the dangers of the disease. She also auctioned her costumes for a cancer charity.
Ms Halliwell will meet the UN Secretary-General, Kofi Annan, in New York. Last week she travelled to Uganda to film a report for Comic Relief.
Her appointment follows a decision by the Tottenham footballer David Ginola to continue work done by Diana, Princess of Wales, to highlight the damage caused by land-mines. "She will be a great ambassador for young people," Alex Marshall, the UN population fund's deputy director for information, said yesterday.
Ms Halliwell, who seems to have been searching for a career since leaving the group, will follow in the footsteps of Audrey Hepburn, Bianca Jagger and the actress Linda Gray. Ms Hepburn, who died in 1993, was awarded an honorary Oscar for her work as ambassador for Unicef. Bianca Jagger used her fame as the former wife of Rolling Stone Mick to highlight the mass rape of Bosnian women.