The girl group are rumoured to have joined the lineup for the London 2012 event on Sunday, August 12.
The performance would be the band's first live performance since 2008, but Mel B has allegedly given organisers a set of instructions.
"The delay has nothing to do with Victoria or Mel C - it's down to Mel B," a source told The Sun.
"She's told organisers that if they want her at the show they are going to have to fork out for a stack of unreasonable demands.
"She wants her friends and family flown from all over to see the gig and wants them to have backstage access. And her rider, which includes a demand for a personal stylist to be flown wherever she goes, is ridiculous."
Victoria Beckham had previously been cited as the one member who was reluctant to reunite for the gig.
A list of performers for the closing ceremony leaked to the media last week, which included Take That, Jessie J, The Who, Elbow and Russell Brand among others.