Britain is seeing daily infections of the rare monkeypox virus that are unconnected to any travel to West Africa, where the disease is endemic, a health official said on Sunday.
The UK Health Security Agency (UKHSA) said new figures would be released on Monday, after it registered 20 cases on Friday.
Asked if community transmission was now the norm in Britain, UKHSA chief medical adviser Susan Hopkins said "absolutely".
"We are finding cases that have no identified contact with an individual from West Africa, which is what we've seen previously in this country," she told BBC television.
"We are detecting more cases on a daily basis."
Hopkins declined to confirm reports that one individual was in intensive care, but said the outbreak was concentrated in urban areas, among gay or bisexual men.
"The risk of the general population remains extremely low at the moment, and I think people need to be alert to it," she said, adding that for most adults, symptoms would be "relatively mild".
The first UK case was announced on May 7, in a patient who had recently travelled to Nigeria. The disease is also spreading in Europe and North America.
Monkeypox can be transmitted through contact with skin lesions and droplets of a contaminated person, as well as shared items such as bedding and towels.
Symptoms include fever, muscle aches, swollen lymph nodes, chills, exhaustion and a chickenpox-like rash on the hands and face. They usually clear up after two to four weeks.
There is no specific treatment but vaccination against smallpox has been found to be about 85 percent effective in preventing monkeypox.
Education Secretary Nadhim Zahawi said the UK government had already started buying up stocks of smallpox vaccine.
"We're taking it very, very seriously," he told the BBC.