A British military healthcare worker who tested positive for the Ebola virus is being flown to the UK for treatment, the Ministry of Defence says.
The woman, who has not been named, was assessed by experts prior to the decision being made to transfer her to the UK for further medical attention.
Her next of kin have been informed.
The MoD said an RAF C17 aircraft had flown to Sierra Leone with an Ebola expert from the Royal Free Hospital in London on board.
An MoD spokeswoman said: "Despite there being stringent procedures and controls in place to safeguard UK service personnel, there is always a level of risk in deployments on operations of this type."
Between 600 and 700 UK defence personnel are currently helping tackle the virus outbreak in Sierra Leone. The British patient is currently at the Kerry Town treatment unit near the capital, Freetown.
A spokesman for Public Health England said: "An investigation in to how the military worker was exposed to the virus is currently under way and tracing of individuals in recent contact with the diagnosed worker is being undertaken.
"Any individuals identified as having had close contact will be assessed and a clinical decision made regarding bringing them to the UK."
He added: "The UK has robust, well-developed and well-tested systems for managing Ebola and the overall risk to the public in the UK continues to be very low."
British nurses Will Pooley and Pauline Cafferkey have both recovered from Ebola after being infected in West Africa last year.
The current Ebola outbreak was first reported in March last year and has killed almost 10,000 people, mostly in the West African countries of Sierra Leone, Liberia and Guinea.