On its most recent U.S. tour, the band played mostly large theaters as a sextet, with Bunton, usually the drummer, taking over Smith's bass and keyboard duties.
Former Spice Girl Emma Bunton and TV presenter Kirsty Gallacher have also been lined up to cover Kelly's holiday absence and the final shows under the GMTV banner.
But 'Posh' did take to the stage with Geri 'Ginger' Halliwell, Melanie 'Scary' Brown, Melanie 'Sporty' Chisholm and Emma 'Baby' Bunton at the end of the performance at London's Piccadilly Theatre.