The UK Health Security Agency (UKHSA) said it had detected two additional cases of monkeypox, one in London and one in the South East, bringing the total confirmed cases in the UK to nine.
The latest cases have no travel links to a country where monkeypox is endemic, so officials believe it is possible they acquired the infection through close contact with an infected person.
Dr Susan Hopkins, Chief Medical Adviser, UKHSA, said: “These latest cases, together with reports of cases in countries across Europe, confirms our initial concerns that there could be spread of monkeypox within our communities.
“UKHSA has quickly identified cases so far and we continue to rapidly investigate the source of these infections and raise awareness among healthcare professionals.”
Dr Hopkins said the agency was advising gay and bisexual men to be aware of “any unusual rashes or lesions” and to contact a sexual health clinic immediately if they had any concerns ahead of a visit.
Monkeypox, which is most commonly associated with travels to countries in west Africa, has not previously been described as a sexually transmitted infection, said officials, though it can be passed on by direct contact during sex.
An urgent probe is now ongoing to figure out how and where the recent cases were acquired, and how they may be linked.
It comes as officials in the US reported a monkeypox case in Massachusetts in a man who recently travelled to Canada.
US health officials are in contact with officials in the UK as part of their investigation but "at this point in time, we don't have any information that links the Massachusetts case to cases in the UK", said Jennifer McQuiston of the US Centers for Disease Control and Prevention (CDC).
The man is in hospital and in a good condition, she added.
Monkeypox typically begins with a flu-like illness and swelling of the lymph nodes, followed by a rash on the face and body.
For many it is a mild, self-limited illness, with most people recovering within weeks, but it is fatal for up to one in ten people, according to the World Health Organisation.