The World Health Organization (WHO) issued an alert Wednesday night warning that more cases of the monkeypox virus could be reported in Britain.
An investigation is underway by British health officials to detect the cause of the outbreak which has hit mainly London, now affecting nine people.
"Based on currently available information, infection seems to have been locally acquired in the United Kingdom. The extent of local transmission is unclear at this stage and there is the possibility of identification of further cases," the WHO added.
Three cases from the same household were initially confirmed on May 6, followed by four more cases a few days later. Two additional cases, one in London and one in the South East of England, were also detected.
"The latest cases bring the total number of monkeypox cases confirmed in England since 6 May to 9, with recent cases predominantly in gay, bisexual or men who have sex with men (MSM)," the UK Health Security Agency (UKHSA) said Wednesday night on its official website.
The four laboratory confirmed cases were reported amongst people who attended a Sexual Health clinic with a vesicular rash illness in gay, bisexual, and other men who have sex with men (GBMSM), the WHO statement said.
Monkeypox is a sylvatic zoonosis with incidental human infections that usually occur in forested parts of Central and West Africa. It is caused by the monkeypox virus which belongs to the orthopoxvirus family, the WHO added.
A detailed contact tracing investigation is also being carried out to determine the likely route of acquisition and establish whether there are any further chains of transmission within Britain for all cases.
"No source of infection has yet been confirmed for either the family or GBMSM clusters," the WHO said.
Susan Hopkins, chief medical adviser at UKHSA, said these latest cases and reports of cases in countries across Europe confirmed their initial concerns that there could be spread of monkeypox within the communities.
"We are contacting any identified close contacts of the cases to provide health information and advice," said Hopkins.
There have been eight previous cases of monkeypox reported in Britain, all linked to a travel history to or from Nigeria.