Ms Halliwell, better known as Ginger Spice, came into conflict with the powerful Catholic Church in the Philippines when she spoke about contraception. "I believe that if you can't control your fertility, you can't control your life, and if you're having sex, you've got to be protected against unwanted pregnancy and infection," she told a crowd outside a clinic run by Marie Stopes International in Manila. "It's everybody's fundamental right."
She said that she thought everyone should practise safe sex by using condoms. Furious Catholic leaders responded swiftly.
"This is a free country, we don't interfere in the right of anybody to go anywhere or say what they believe [but] we do not need population control, and any effort at safe sex is totally, utterly immoral from top to bottom," said the Rev James Reuter, the director of the National Office of Mass Media of the Roman Catholic Church of the Philippines.
Ms Halliwell, 27, chose the Philippines as her first assignment because of its high rate of population growth and a large population of unmarried mothers. With 74 million people, the Philippines has a population growth rate of 2.32 per cent, one of the highest in the world. After visiting the clinic she went to a health centre funded by the United Nations in one of Manila's slums.
Satish Mehra, the UN Population Fund's representative in the Philippines, said Ms Halliwell's fame would help the UN's awareness campaign for reproductive health. "I'm sure she will make an impact. She has a large following around the world and in the Philippines and I'm sure young people will listen to her." he said.
"She's a natural in making contacts with everyone she meets," said Patricia Hindmarsh, the director of Marie Stopes International, a world-wide private charity that promotes contraceptive use and reproductive health and advises the UN Population Fund on reproductive health care.
"We hope that the experience Geri gains during her visit will help her influence governments and donors of the need for quality reproductive health services in the Philippines and everywhere," Ms Hindmarsh added.
At the beginning of her trip, Ms Halliwell admitted that she was not a reproductive health expert. "I know I've got a lot to learn but what better way than to go and see for myself what the United Nations Population Fund and Marie Stopes International are doing at a grassroots level.
"What's important about this trip is that it gives me the chance to meet the people who really matter - women who want to control their fertility and young people who are on the brink of making important decisions about their sexual health," she said.
Ms Halliwell is pursuing a solo career after leaving the group in June last year because of differences with her fellow members.
She recently launched a single called "Look at Me", which reached number two in the British charts, and has had disappointing sales with her album Schizophonic.