The hospital said in a statement Wednesday that Pauline Cafferkey is now critically ill after her condition deteriorated.
Cafferkey was treated for Ebola after returning from Sierra Leone last year. She was released from the hospital's high-level isolation unit in January but she suffered a relapse last week and was flown by military plane into London for treatment.
Experts say evidence is growing that even among those who survive Ebola, mental and physical health problems can last for years even after the virus is cleared from the bloodstream.
Thousands of Ebola survivors in West Africa are now suffering from long-term side effects of the often-deadly disease.