The UK Health Security Agency (UKHSA) said a person who had recently travelled to West Africa had contracted a strain of the virus distinct from that currently circulating in Britain.
Contact tracing of the individual is underway, the UKHSA said. It is not clear how the person caught monkeypox or whether they are male or female.
The individual has been admitted to the Royal Liverpool University Hospital for further testing, the UKHSA said.
There are two distinct strains of the monkeypox virus, known as Clade I and Clade II.
Clade I has a mortality rate of around one in 10, while the fatality rate for Clade II is around one in 100.
The latest UKHSA figures show there are just over 3,100 confirmed monkeypox cases in the England, with around 70 per cent of infections in London. There were 25 new cases in the capital in the week ending August 30, bringing the total to 2,257.
Dr Sophia Maki, Incident Director at the UKHSA, said: “UKHSA and the NHS have well established and robust infection control procedures for dealing with cases of imported infectious disease and these will be strictly followed and the risk to the general public is very low.
“We remind everyone who is planning to travel to West and Central Africa to be alert for the symptoms of monkeypox and to call 111 if you have symptoms on your return.”
The original outbreak of monkeypox last May raised alarm among health officials because the viral disease, which was first found in monkeys, mostly occurs in west and central Africa and only occasionally spreads elsewhere.
It comes just days after health officials said that transmission of the virus appears to be slowing in Europe.
Dr Hans Kluge, the World Health Organisation’s regional director for Europe, told a briefing on Tuesday that there were “encouraging early signs” that the outbreak may be slowing in the UK, Germany and France.
There were 79 monkeypox cases recorded in the UK in the past week, down from 126 the week before and 167 the week before that.
Symptoms of the virus include fever, headache, muscle aches, backache, swollen lymph nodes, chills and exhaustion.
A rash can also develop, often beginning on the face and spreading to other parts of the body. Scabs can form on the body which then fall off.
Monkeypox is not normally a sexually-transmitted infection, but it can be passed on by direct contact during sex.
It can also be spread through touching clothing, bedding or towels used by someone with the monkeypox rash, and through the coughs and sneezes of somebody with the infection.
Scientists at the University of Oxford last week launched a clinical trial to assess whether Tecovirimat – also known as Tpoxx – can help reduce the amount of time people are sick.