Another linked case has been idenitfied in the north-east of London, health bosses said on Monday.
Investigations are under way to establish links between the latest four cases, who all appear to have been infected in London, the UK Health Security Agency (UKHSA) said.
All four of these cases self-identify as gay, bisexual or other men who have sex with men, the agency added.
Currently, common contacts have been identified for two of the four latest cases.
There is no link to travel to a country where monkeypox is endemic, and exactly where and how they acquired their infections remains under urgent investigation, including whether they have further links to each other.
Those patients needing medical care are all in specialist infectious disease units 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 that there are currently seven confirmed monkeypox cases in the UK, diagnosed between May 6 and 15.
Due to the recent increase in cases and uncertainties around where some of these individuals acquired their infection, the UKHSA said it is working closely with NHS partners to identify if there may have been more cases in recent weeks, as well as international partners to understand if similar rises have been seen in other countries.
Dr Susan Hopkins, chief medical adviser at the UKHSA, said the cases were “rare and unusual”.
She added: “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.
“We are particularly urging men who are gay and bisexual to be aware of any unusual rashes or lesions and to contact a sexual health service without delay.
“We are contacting any potential close contacts of the cases to provide health information and advice.”
The health agency said initial symptoms of the viral infection include fever, headache, muscle aches, backache, swollen lymph nodes, chills and exhaustion.
It said a rash can develop, which changes and goes through different stages before finally forming a scab, which later falls off.
Monkeypox is usually a mild self-limiting illness 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.