"Swollen vocal cords" forced Demi Lovato to postpone her show at the O2 in London hours before she was due to appear on stage.
In a statement on social media, written through "tears of frustration and sadness", she added that her vocal cords could have been injured had she performed, potentially threatening the rest of her tour.
She has seen an ENT doctor and is "very, very sick", the American singer-songwriter said.
The Sunday night show has been rescheduled for 25 June.
Lovato told her supporters it broke her heart to let them down, and that she was "so so sorry to disappoint any of my fans in London that have travelled hours to be here by plane or car".
She would not have postponed unless she "absolutely had to".
Concert producer Live Nation said: "Unfortunately due to illness, tonight's Demi Lovato show at the O2 has had to be postponed.
"The show will now take place on Monday 25 June. All tickets will remain valid for the new date.
"Please contact your point of purchase for all ticket inquiries."