A total of 37 more cases of monkeypox have been detected in England and Scotland, public health officials said on Monday, taking the total to 57.
Thirty-six confirmed cases were found in England, and one north of the border, the UK Health Security Agency and Public Health Scotland said.
The UKHSA said it was now advising high-risk contact cases of confirmed cases who have not tested positive or developed symptoms to isolate for up to 21 days.
It has also bought supplies of smallpox vaccine, which is being offered to close contacts to reduce the risk of symptomatic infection and severe illness.
Chief medical adviser Susan Hopkins said contact tracing was helping to limit the close-contact spread of the virus, which causes a chickenpox-like rash.
"Because the virus spreads through close contact, we are urging everyone to be aware of any unusual rashes or lesions and to contact a sexual health service if they have any symptoms," she added in a statement.
"A notable proportion of recent cases in the UK and Europe have been found in gay and bisexual men so we are particularly encouraging these men to be alert to the symptoms."
The first case of monkeypox in the UK was announced on May 7 in a patient who had recently returned from Nigeria, where the disease is endemic.
Health officials said last week that further cases were not connected to travel and there appeared to be community transmission.
The risk to the general population, however, remained extremely low, they added.