But rather than getting angry, the fans who'd arrived at the venue got together to send a message to the singer, who'd taken ill earlier in the day.

Gathering at the Peninsular Spire in Greenwich, just outside the O2, they held up "get well soon" signs and sang a chorus of Lovato's hit Skyscraper.

The video found its way to the singer, who said the gesture "made me cry".

"My fans are seriously the best," she added. "I can't believe how much love and support I've received today."

Lovato, who is suffering from swollen vocal cords, postponed Sunday night's show just 45 minutes before the doors were due to open.

Announcing the news on Twitter, she said she had consulted a doctor, who said she could cause permanent damage to her voice if the concert went ahead,

"It breaks my heart to let you down. Just know I feel your frustration and I wouldn't do his unless I absolutely had to," she added.

The concert has been rescheduled for 25 June.

The star, whose hits include Sorry (Not Sorry) and Cool For The Summer, is due to play Birmingham on Tuesday, with further dates in Glasgow, Newcastle upon Tyne and Manchester.

No announcement has been made on whether these shows will go ahead.