Another linked case has been identified in north-east London, health chiefs said Monday.
Investigations are underway to establish links between the latest four cases, all of which appear to be infected in London, the UK Health Security Agency (UKHSA) said.
All four of these cases identify themselves as gay, bisexual or other men who have sex with men, the agency added.
Currently, common contacts have been found for two of the last four cases.
There is no link to traveling to a country where monkeypox is endemic, and exactly where and how they contracted their infections is under urgent investigation, including whether they have any further links to each other.
Those patients in need of medical care are all in specialist infectious disease wards at the Royal Free Hosptial, Royal Victoria Infirmary in Newcastle upon Tyne and Guys’ and St Thomas’.
The individuals have the West African clade of the virus, which is mild compared to the Central African clade.
These latest cases mean there are currently seven confirmed cases of monkeypox in the UK, diagnosed between 6 and 15 May.
Due to the recent increase in cases and uncertainties about where some of these individuals contracted their infection, the UKHSA said it is working closely with NHS partners to determine whether there may have been more cases in recent weeks, as well as international partners to understand whether similar increases have been observed in other countries.
dr. Susan Hopkins, chief medical officer at the UKHSA, said the cases were “rare and unusual”.
She added: “UKHSA is rapidly investigating the source of these infections as the evidence suggests there may be transmission of the monkeypox virus in the community, spread through close contact.
“We particularly urge men who are gay and bisexual to be aware of any unusual skin rashes or lesions and to contact a sexual health service immediately.
“We are contacting all possible close contacts of the cases to provide health information and advice.”
The health service said the first symptoms of the viral infection are fever, headache, muscle aches, back pain, swollen lymph nodes, chills and exhaustion.
It said a rash can develop, which changes and goes through several stages before eventually forming a scab, which later falls off.
Monkeypox is usually a mild, self-limiting disease and most people recover within a few weeks.
The virus does not spread easily between people and the risk to the UK population is low.