The singer has been suffering from a chest infection for the past two months, and has been unable to get fit for rehearsals starting next month, a source revealed.
"Victoria can't get rid of her infection," The Sun quoted the source, as saying.
"She's worried she won't be fit while all the other girls are turning their bodies into machines."
Posh is the latest one with the injury scare.
Hubby David Beckham was out for weeks following a knee injury. Dad-in-Iaw Ted also gave them a scare when he suffered a heart attack earlier this month.