Emma Bunton said she has cancelled two performances in Birmingham and York due to illness.
The singer, 46, whose nickname in the Spice Girls was Baby Spice, has been touring for a series of Christmas shows called Emma Bunton And Friends.
She told her Twitter followers on Saturday: “I feel like I have let so many of you down today & it has left me heartbroken.
“With rest & medication I wake every morning with hope. Birmingham I am here but unfortunately won’t be able to perform tonight under doctors orders.”
Bunton added: “I’m overwhelmed by all the support & love from my fans. Love you all very much.”
She is next due to play the Theatre Royal Drury Lane in London on Monday, but has not said whether this concert will go ahead.
She also told her followers on Friday: “Please accept my sincere apologies, my Christmas Show in York cannot go ahead tonight.
“I have been advised by my doctor that I am unable to perform. I am absolutely devastated and (I’ll) see you all again very soon. ХХ.
“I have never in my whole career ever had to cancel a gig and I can’t stop crying.”
Her show is described as building on the success of a Royal Albert Hall Christmas show by Bunton in 2019 which was attended by Myleene Klass, Laura Whitmore, Holly Willoughby, Vanessa Feltz and Christine Lampard.
The recent show sees her “performing songs from her own back catalogue as well as some classic Spice Girls tracks” as she is joined by “special guests”, which included a performance by singer Will Young and was attended by fellow Spice Girl Geri Horner.
The Spice Girls released a new and expanded edition of their 1997 album Spiceworld, in celebration of the band’s 25th anniversary, called Spiceworld 25, in November.
In 2019, the group, minus Victoria Beckham, got back together for a string of tour dates across the UK and Ireland.
It was the first time the Spice Girls had performed together since the 2012 London Olympics closing ceremony.
