The 24-year-old singer was due to fly into the British capital today but her management axed her flight and have refused to comment on the issue, reported Daily Mail.
"She cancelled her trip and all work commitments in the UK at the very last minute, which is very unlike her. She has been hitting it hard for months. She smokes, drinks and parties hard on top of work commitments," a source said.
The 'We found love' hitmaker was last seen out in Los Angeles on June 5 night looking rather tired when she came out to grab a drink.
It was only last month that she was hospitalised due to exhaustion and dehydration.