The singer was pictured looking tired and dishevelled as he walked through London's Heathrow Airport in the early hours of Friday (20Mar15) after arriving on a flight from Asia.

Malik pulled out of a Coca-Cola commercial shoot in Hong Kong on Thursday (19Mar15) and jetted back to his native U.K., with a spokesman confirming the star had left the One Direction tour and been "signed off with stress".

The singer's mother, Trisha, has since spoken out to insist her son is doing OK, telling Britain's Daily Mirror newspaper, "I have spoken to him and he's fine. We have been in contact."

The drama came just days after Malik was forced to deny allegations he had cheated on his pop star girlfriend, Little Mix's Perrie Edwards, after he was pictured partying with another woman in Thailand.

The band is expected to continue the tour without Malik.