All identified cases are from the same family in the east of England and are linked to recent travel to west Africa, the UK Health Security Agency (UKHSA) said.
Two cases were announced earlier in the week, with a third under investigation. UKHSA has now confirmed their diagnosis but reported that the patient had died as a result of the acute viral illness.
A Bedfordshire Hospitals NHS Foundation Trust, which was treating the individual, said: “We confirm the sad death of a patient at our trust, who had confirmed Lassa fever. We send our deepest condolences to their family at this difficult time.
“We will continue to support the patient’s family and our staff and are working closely with colleagues from the UK Health Security Agency to undertake a robust contact tracing exercise.”
Since 1980, there have been eight cases of Lassa fever imported into the UK, with the last two cases occurring in 2009.