Three of the cases have been detected in London, and one in the North East of England, the UK Health Security Agency (UKHSA) announced on Monday (May 16).
The health agency said it was working to find links between the latest four cases, which all appear to have been infected in the capital.
Common contacts have been established between two of the four individuals who have caught the virus.
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Those needing care are being treated in specialist infectious disease units at the Royal Free Hospital, Royal Victoria Infirmary in Newcastle upon Tyne and Guy’s and St Thomas’.
The new cases do not have any known links with two other cases confirmed on May 14 or another case announced on May 7.
Dr Susan Hopkins, chief medical adviser at the UKHSA, said: “This is rare and unusual.
“UKHSA is rapidly investigating the source of these infections because the evidence suggests that there may be transmission of the monkeypox virus in the community, spread by close contact.”
The first case was a person who had recently travelled to Nigeria, which is where they were believed to have contracted the infection, before travelling to the UK.
All four of the new cases self-identify as gay, bi-sexual or other men who have sex with men, the UKHSA said.
The health body is therefore asking these groups “to be alert” to possible symptoms, which include rashes or lesions on any part of their body, and to contact a sexual health service if they have concerns.
The UKHSA said it was working closely with NHS partners to establish whether there have been any more cases in recent weeks, and international partners to examine whether other countries have seen a similar rise in monkeypox.
The health agency emphasises that the virus does not spread easily between people and the risk to the UK population is low.
What is monkeypox?
A rare disease caused by a viral infection.
How does it spread?
Most commonly when a person comes into close contact with an infected animal.
It is not spread easily between people.
But it can be spread through:
- touching clothing, bedding or towels used by someone with the monkeypox rash
- touching monkeypox skin blisters or scabs
- the coughs or sneezes of a person with the monkeypox rash
What are the symptoms?
Infected people usually start to show symptoms between five and 21 days after infection.
These include fever, headache, muscle aches, backache, swollen lymph nodes, chills and exhaustion.
A rash can develop, often beginning on the face, then spreading to other parts of the body.
The rash changes and goes through different stages – a bit like chicken pox –before finally forming a scab, which later falls off.
How serious is it?
Most patients recover within a few weeks and do not need treatment, but it can cause severe illness in some people.
Why is it called monkeypox?
The disease was first discovered in monkeys kept for research in 1958.
The first human case was recorded in 1970 in the Democratic Republic of Congo.