The singers became embroiled in the challenge when their former bandmate Melanie Chisholm recently dared Brown to strip naked and perform a dance on the balcony of her California home.
Brown completed the task, and told Chisholm and Halliwell to go one better by singing Spice Girls song Say You'll Be There in a pub.
Halliwell has now completed the dare and posted video evidence online.
In the footage, Halliwell is seen walking into a quiet pub and bursting into song in front of baffled punters, who pretend to ignore the star, before she turns to the camera and says, "Done it! Melanie - it's your turn!"
In a post on her blog, Halliwell reveals she felt "liberated" by acting "foolish" in public, writing, "I try to be a woman of my word - if I say I'm going to do something, I like to do it... Hey, I've done the Olympics (sang at the closing ceremony), this will be easy-peasy! I walk in, I sing, I'm doing it... Eeek!... I feel silly - but it's funny!... I feel liberated! Being foolish and sharing it feels real rather good. I was reminded again thanks to my soul sista (sic) Mel B."
She goes on to give Brown a new challenge, adding, "Now then, a dare for Mel B?... A visit to the Hollywood dentist! Start licking the dentist's finger when he puts finger in mouth - Too dirty? Oh yeah, and I want evidence!"