A British national living in Sierra Leone has tested positive for Ebola, the Department of Health has said.
It is the first reported case of a Briton contracting the deadly virus during the recent outbreak.
Some 1,427 people have died since Ebola began spreading in West Africa at what the World Health Organization (WHO) has described as an "unprecedented" rate.
The Department of Health (DoH) said the overall risk to those in the UK continued to be "very low".
The number of cases in Africa since the outbreak began in March now stands at 2,615.
The virus, which is spread between humans through direct contact with infected blood, attacks internal organs and has no known cure.
Symptoms include high fever and bleeding, and up to 90% of cases result in death.
In a statement, the DoH said medical experts were "assessing the situation to ensure that appropriate care is delivered".
Consular assistance is being provided, the statement added.
No further details have been given about the British national.
A decision will now have to be made over whether the patient should be flown back to the UK, said BBC Nigeria correspondent Will Ross.
Earlier this month, British Airways suspended flights to Sierra Leone and Liberia until at least the end of August.
The Foreign Office has said Britons should think carefully before travelling to Sierra Leone, Guinea or Liberia.
"General medical facilities throughout Sierra Leone are currently under severe strain due to the Ebola outbreak, and unable to provide the same standard of healthcare as in the UK," current advice states.
"Dedicated healthcare facilities for Ebola are overwhelmed."
Deputy chief medical officer Prof John Watson said: "The overall risk to the public in the UK continues to be very low.
"We have robust, well-developed and well-tested NHS systems for managing unusual infectious diseases when they arise, supported by a wide range of experts."
Sierra Leone is bordered by Guinea to the north east and Liberia to the south east.
Ivory Coast, the largest economy in francophone West Africa, had previously imposed a ban on flights to and from Sierra Leone, as well as Liberia and Guinea.
Ebola Virus Disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Fatality rate can reach 90% - but current outbreak has mortality rate of about 55%
- Incubation period is two to 21 days
- There is no vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats, a delicacy for some West Africans, are considered to be virus's natural host
On Friday, it was confirmed that an Irish engineer who died at home after returning from Sierra Leone did not have the virus.
And during the Commonwealth Games in Glasgow, athletes from Sierra Leone were checked for the disease, but tested negative.
On Thursday, two US doctors were discharged from hospital after being given an experimental drug, ZMapp, while three Liberian medics are also recovering well.
The current outbreak in West Africa is the deadliest occurrence since Ebola was discovered in 1976.
The WHO has declared a public health emergency and has warned the scale of the current outbreak has so far been underestimated.
As a result, the Sierra Leonean parliament passed a new law meaning anyone harbouring Ebola victims could face two years in prison.