The rare viral infection was recorded on Friday in a Nigerian national staying at a naval base in Cornwall, Public Health England (PHE) said.
United Kingdom officials are now working to track down and notify people who may have been in close contact with the patient, including some people who traveled on the same flight as the patient to the United Kingdom.
The rare viral infection does not spread easily between humans and most people recover within a few weeks.
It is commonly a self-limited disease with the symptoms lasting from 2 to 3 weeks. There have been scattered cases throughout Africa and Asia since then, with Nigeria reporting three cases in the mid 1970s.
The patient, whose identity has not been revealed, was transferred to the expert infectious disease unit at the Royal Free Hospital in London on Saturday morning.
Dr Michael Jacobs, clinical director of infection at the Royal Free Hospital, said monkeypox "does not spread easily between people and the risk of transmission to the wider public is very low".
Dr Nick Phin who is the deputy director of the National Infection Service said PHE "is following up those who have had close contact with the patient to offer advice and to monitor them as necessary".
Monkeypox is a mild self-limiting illness causing symptoms such as fever, headache, muscle aches, swollen lymph nodes, exhaustion and, in some cases, a rash. Soon after, patients develop a "pox" rash, with lesions that often appear over the face and trunk.
People infected with monkeypox can spread the disease to others, mostly through "large respiratory droplets" that are expelled when a person coughs, sneezes or talks, according to the Centers for Disease Control and Prevention (CDC). The virus lives in animals, including primates and rodents, but can sometimes "jump" from animals to people, the World Health Organization says. Both health departments have well established and robust infection control procedures, that deal with cases of imported infectious diseases and are following strict rules to minimize the risk of transmission.
Monkeypox was first discovered in crab eating Monkeys in Copenhagen, Denmark in 1958 by a scientist called Preben von Magnus.
Monkeypox was first identified in humans in 1970 in the Democratic Republic of Congo when a nine-year-old boy was diagnosed. It eventually forms a scab which falls off.