There are four additional cases of monkeypox, three of which are in London, the UK Health Security Agency (UKHSA) has confirmed.
The UKHSA says all the patients are gay or bisexual men, and urged gay men to be aware of any unusual rashes and lesions.
The total UK confirmed cases is now seven, diagnosed between May 6 and 15.
The UKHSA said the new cases do not have known connections with the previous three infections, as it warned there was evidence of community transmission in the UK of the disease.
The patients are being treated at specialist infectious disease units at the Royal Free Hospital in Camden and Guys’ and St Thomas’ in Westminster.
Investigations are underway to establish links between the latest four cases, who all appear to have been infected in London.
The health agency said two of the four cases know each other, and all patients identify as gay, bisexual or other men who have sex with men.
The UKHSA says anyone with concerns that they could be infected should see a health professional, but make contact with the clinic or surgery ahead of a visit.
None of the newly infected individuals have travelled to a country where monkeypox, a virus which kills up to 10% of people it infects, is endemic, UKHSA said.
Dr Susan Hopkins, chief medical adviser, 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.
“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.”
What is monkeypox?
Monkeypox can be caught from infected wild animals in parts of west and central Africa.
It’s thought to be spread by rodents, such as rats, mice and squirrels.
You can catch monkeypox from an infected animal if you’re bitten or you touch its blood, body fluids, spots, blisters or scabs.
It may also be possible to catch monkeypox by eating meat from an infected animal that has not been cooked thoroughly, or by touching other products from infected animals
According to the NHS, it’s very uncommon to get monkeypox from a person with the infection because it does 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, or through the coughs or sneezes of a person with the monkeypox rash.
Early symptoms of monkeypox include fever, headache, muscle aches, backache, swollen lymph nodes, chills and exhaustion.
A rash can also develop, usually starting on the face before spreading to other areas of the body.
The rash progresses through different states until it forms a scab which falls off.
Symptoms usually clear up within two to four weeks.