Two people in the East of England have been diagnosed with Lassa fever, an acute viral illness, after travelling to West Africa.
A third "probable case" is also under investigation and being treated at Bedfordshire Hospitals NHS Foundation Trust, the UK Health Security Agency (UKHSA) said.
The agency said the cases were within the same family in the East of England.
Most people make a full recovery but severe cases can be fatal.
People usually become infected with Lassa virus through exposure to food or household items contaminated with the urine or faeces of infected rats.
The virus can also be spread through infected bodily fluids.
The UKHSA said one of the confirmed cases had recovered and the other would receive specialist care at the Royal Free London NHS Foundation Trust.
The High Consequence Infectious Disease Network was engaged with their ongoing care, it said.
What is Lassa fever?
- Lassa fever, like Ebola, can be spread through contact with the bodily fluids (blood, saliva, urine or semen) of infected people
- Humans can also get it by coming into contact with the urine or faeces of infected rodents that carry the disease
- The disease typically causes a fever and flu-like symptoms, but it can cause bleeding through the nose, mouth and other parts of the body
- Most people will make a full recovery but the illness can be fatal
- Described as a cousin of Ebola, the disease is endemic in a number of West African countries
Dr Susan Hopkins, chief medical advisor at UKHSA, said: "Cases of Lassa fever are rare in the UK and it does not spread easily between people.
"The overall risk to the public is very low.
"We are contacting the individuals who have had close contact with the cases prior to confirmation of their infection, to provide appropriate assessment, support and advice."
Since 1980, there have been eight cases of Lassa fever imported into the UK, with the last two cases occurring in 2009.
The UKHSA said: "There was no evidence of onward transmission from any of these cases."