The band, who are midway through their 17-date 'Headlines!' tour, apologised for King's absence at their Oxford gig via Twitter after doctors advised her not to perform.
They wrote: "Hey guys - we're really sorry Mollie couldn't join us on stage in Oxford tonight.
"She was very unwell and her doctor told her she couldn't perform. Get well soon Molls! We love you loads and really missed you. (sic)"
Vanessa White later added: "Had a great time in Oxford tonight but [Mollie] was HUGELY missed!! Get well soon babe mwah xxxxx"
The group are due to perform in Leicester's De Montfort Hall this evening.
> Click here to read our review of The Saturdays' 'Headlines!' tour.