The pop beauty had been due to perform at The O2 Arena tonight as part of her Tell Me You Love Me World Tour.
However, at 5:17 PM she pulled the plug on the gig.
Taking to Twitter, Demi said: “I write this through tears of frustration and sadness that I have to postponed my concert tonight due to being very, very sick and with swollen vocal cords.
“If I go on tonight and injure my vocal cords any more I could potentially have to cancel the rest of the tour.
“I’m so, so sorry to disappoint any of my fans in London that have travelled hours to be here by plane or car and to all of the fans that got these tickets for a special occasion.”
Meanwhile, a statement from The O2 Arena read: “Unfortunately, due to illness, tonight’s Demi Lovato show has had to be postponed. The show will now take place on Monday June 25th. All tickets will remain valid for the new date.
“Please contact point of purchase for all ticket enquiries”
As news broke gutted fans reacted with anger, furious at the uber late decision to axe the show.
“I’m so gutted for anyone that travelled all the way to London today/tonight for Demi. A very last minute cancellation is very unprofessional,” one music lover tweeted.
Another added: “Demi Lovato cancelling her concert one hour before doors open is so out of order.
“It’s understandable that she’s sick but people have travelled for hours. London isn’t just a 20 minute drive for everyone.”
“I already in the queue for the bloody admission gate,” another fumed.
“Why have you waited until half hour before the doors open to announce this?????” a sad fan wrote.
Less than 24 hours before the cancelation Demi made a sursprise appearance at Captial FM’s Summertime Ball.
The chart-topper joined Clean Bandit on stage to perform their latest single Solo.