The 21-year-old singer, who was hospitalised on 15 April for a "severe allergic reaction", has had a setback in her recovery with the return of her symptoms.
It has meant that she has been forced to postpone to start of her European tour — weeks after the same illness derailed her US tour.
"Cyrus is continuing to recover from the severe allergic reaction she suffered on April 15," Miley's representative confirmed.
"After a period of improvement, Cyrus' symptoms have returned and doctors will not permit her to travel. This type of extreme reaction can last up to 27 days and Cyrus must remain under Doctor’s care at this time."
Miley has been forced to postpone her European tour
The pop star is still hoping to perform her Bangerz show at London's O2 Arena on 6 May.
Miley initially became ill with flu-like symptoms in early April, causing her to pull out of her show in Charlotte, North Carolina. She was prescribed the antibiotic Cephalexin, but an allergic reaction to the drug left her hospitalized for more than a week.
Miley was first hospitalised on 15 April
"It was so scary," she told Ryan Seacrest in a radio interview on Monday. "I had basically been poisoning myself with something I didn't know I was really scary allergic to."