Victoria Beckham, Geri Halliwell, Emma Bunton, Mel C and Mel B played a medley of 'Wannabe' and 'Spice Up Your Life' at the show last night (August 12).
The girl group had previously reunited but did not perform at the launch of the Viva Forever! musical last month.
Ladbrokes has dropped its odds on the Spice Girls getting the Christmas number one from 66/1 to 25/1.
Alex Donohue of Ladbrokes said: "We reckon the Christmas number one bets for the Spice Girls were more in hope than anything else.
"It would be silly of us to completely rule out a comeback after that performance however."
Odds on the group announcing a formal reunion are now 10/1, while the girls are 20/1 to record new material in 2012.
The Spice Girls can be backed for as short as 3/1 to perform together with their full lineup before the end of the year.
London 2012 closing ceremony - photo gallery:
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