Demi Lovato pushed back her gig in London after doctors deemed her unfit for performance due to "swollen vocal chords".
The 25-year-old singer shared a lengthy post on Twitter where she apologised to her fans for postponing the show.
"I write this through tears of frustration and sadness that I have to postpone my concert in London at the O2 tonight due to being very, very sick with swollen vocal chords that were examined by an ENT (dr.). If I go on tonight and injure my vocal chords anymore I could potentially have to cancel the rest of my tour.
"I'm so so sorry to disappoint any of my fans in London that have traveled hours to be here by plane or car, and to all of the fans that got these tickets for their birthday, holiday, or any other special occasion. It breaks my heart to let you down (sic)" she wrote.
The "Sorry Not Sorry" hitmaker promised to reschedule the concert for June 25 and said she is deeply sorry for the "frustration" caused by the late cancellation.
"Just know I feel your frustration and wouldn't do this unless I absolutely had to. For everyone who bought a ticket to tonight's show, I will see you for the rescheduled date on June 25th after Madrid and Rock in Rio. Thank you for understanding. I love you," Lovato added.