Demi Lovato canceled her concert Sunday after experiencing swollen vocal cords.

The 25-year-old singer apologized to fans on Twitter after rescheduling her show at The O2 Arena in London less than hour before she was to take the stage.

"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 and with swollen vocal cords that were examined by an ENT (dr.)," Lovato wrote.

"If I go on tonight and injure my vocal cords anymore I could potentially have to cancel the rest of my tour."

"I'm so so so sorry to disappoint any of my fans in London," she said. "It breaks my heart to let you down. Just know I feel your frustration and I wouldn't do this unless I absolutely had to."

Lovato rescheduled the concert for June 25. She said in a later tweet that she was moved to tears by her fans' dedication and understanding.

"My fans are seriously the best... I can't believe how much love and support I've received today.. the video of my fans singing skyscraper outside of the venue made me cry," the star said.

Lovato kicked off her Tell Me You Love Me world tour in February. She is scheduled to perform Tuesday at Arena Birmingham in Birmingham, England.

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"I'll definitely be more up close and personal with the audience than I have been before," the singer said of the tour in an interview with Billboard in February.

"It will represent who I am as a person and an artist and I can't wait for this country to see it," she added.

Photo credit: Demi Lovato/Instagram