The 'Viva Forever' hitmakers - who were known as Ginger and Sporty Spice respectively during the girl group's heyday in the 90s - jumped on stage together at the last minute at the NSPCC and Childline fundraiser at Annabel's in London on Tuesday night (26.06.18).
In a clip of them singing halfway through, Geri says: ''Aww, it's so nice to sing with you again.''
And Mel joked: ''This was not rehearsed, I am sure you can't tell.''
Mel dominated the song with Geri occasionally joining in.
Geri - who attended with her husband Christian Horner - and later shared a series of photos from their performance on her Instagram Story.
Alongside the first one, she wrote: ''This happened. Spontaneously! [sic]''
The 45-year-old singer added that getting to perform with Geri again ''warmed [her] heart''.
She captioned a picture of them hugging on stage: ''Melc & me And 2 became ! Warmed my heart [sic]''
However, Victoria and Mel C performed the 1996 hit from their self-titled album on New Year's Eve last year in the Maldives.
As far as a full reunion goes, Emma recently teased their plans.
She said: ''There is definitely something in the pipeline, it's so difficult because we want it to be the right thing and I'm sure Victoria will be involved in some way.
''A tour? I'm not sure, but I think a few shows would be great. It would be nice to headline Glastonbury.
''The opportunities are amazing, I was with Geri and Mel C in Mel's kitchen and we're like, 'What are we going to do?' There's so much excitement it's just about making sure we do the right thing.''
Whilst Mel B recently claimed that the 'Stop' hitmakers will be back on stage by September.
She said: ''The deal is done and all five of us are very much looking forward to taking to the stage again.''
The Spice Girls last toured after reuniting in 2007, with their greatest hits shows wrapping up in 2008. The last time they performed as a five-piece was at the 2012 London Olympics closing ceremony.
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