Pauline Cafferkey, 39, was transferred from the Queen Elizabeth University Hospital in Glasgow to an isolation unit at the Royal Free Hospital in London early on Friday morning, the Royal Free said in a statement.
"Pauline Cafferkey is in a serious condition," the hospital said, adding that she was suffering from "an unusual late complication".
Cafferkey, from South Lanarkshire, Scotland, spent several weeks in an isolation unit at the Royal Free at the beginning of the year after contracting the virus in December 2014.
She was the first person to have been diagnosed with Ebola on British soil. She was discharged in January after seemingly making a full recovery.
The Ebola virus can only be transmitted by direct contact with the blood or bodily fluids of an infected person while they are symptomatic. The Royal Free said the risk to the general public remained low.
