LONDON (AFP) - A female British military healthcare worker who contracted Ebola in Sierra Leone arrived in hospital in London on Thursday after she was flown back from the West African country.

The third Briton to contract the virus in Sierra Leone, the health worker was taken by Royal Air Force (RAF) plane to be treated in an isolation unit at London's Royal Free Hospital.

The condition of the patient, who on Wednesday was publicly confirmed to have Ebola, is unknown. She was exposed to the virus while treating patients.

The RAF plane also brought back two of her colleagues for monitoring.

"Two further healthcare workers returning from Sierra Leone will also be assessed at the Royal Free," the hospital said.

"Neither of these individuals has been diagnosed with the Ebola virus and neither is displaying symptoms of the disease."

Public Health England (PHE), a government agency, has said that the risk to the British public was "very low."

Two more military healthcare workers identified as close contacts of the woman who has contracted the virus are to return to Britain on Friday.

They will be flown back on separate European Union medevac flights to Newcastle in northeast England, and taken to the city's Royal Victoria Infirmary.

"They will be initially assessed in hospital and a decision will be made whether they need to be admitted or discharged to appropriate accommodation where they will be monitored for any symptoms for the remainder of their incubation period," PHE said.

The two previous British healthcare workers to contract Ebola - nurses William Pooley and Pauline Cafferkey - both recovered after being treated at the Royal Free.

The hospital has a high-level isolation unit geared up for treating Ebola patients under quarantine.

University of Reading virologist Ben Neuman said the patient would likely be given "antibody-rich serum from Ebola survivors to knock down the amount of virus in her blood while her immune system is learning to fight Ebola".

"The Royal Free Hospital has a 100 percent record in treating Ebola cases so far. Let's hope that doesn't change," Neuman said.

Ebola has swept through Sierra Leone, Guinea and Liberia since December 2013, killing more than 9,900 people, according to the World Health Organization.

Britain has focused its aid effort on Sierra Leone, a former colony, and up to 700 British military personnel have been deployed along with numerous civilian medical staff.

A Ministry of Defence 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."