Four more cases of rare viral monkeypox infection have been diagnosed in England, health authorities say, taking the total confirmed cases since May 6 to seven.

The UK Health Security Agency (UKHSA) said investigations were underway to establish links between the latest four cases who appear to have been infected in London and do not have known connections with the other three cases.

The latest four cases have no links to travel to a country where monkeypox is endemic and exactly where and how they got infected remains under investigation, including whether they have further links to each other, the agency said.

Monkeypox is a rare viral infection similar to human smallpox, which was eradicated in 1980.

Although monkeypox is much milder than smallpox, with most infected people recovering within a few weeks, it can in rare cases be fatal.

All four of the new cases self-identify as gay, bi-sexual or other men who have sex with men, the UKHSA said.

They have the west African clade of the virus, which is mild compared to the Central African one, it said.

"This is rare and unusual," UKHSA chief medical officer Susan Hopkins said in the statement.

"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."

She said the agency is 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.