The UK Health Security Agency said on Friday it had detected 11 new cases of monkeypox in England, taking the total number of confirmed cases in the country to 20.
Britain had previously reported a total of nine cases of the usually mild viral illness, characterised by symptoms of fever as well as a distinctive bumpy rash.
"We expect this increase to continue in the coming days and for more cases to be identified in the wider community. Alongside this we are receiving reports of further cases being identified in other countries globally," UKHSA Chief Medical Adviser Susan Hopkins said in a statement.
"We continue to rapidly investigate the source of these infections and raise awareness among healthcare professionals."
British Health Secretary Sajid Javid said he had updated G7 health ministers on what is known so far about the outbreak.
"Most cases are mild, and I can confirm we have procured further doses of vaccines that are effective against monkeypox," Javid said on Twitter.
There isn't a specific vaccine for monkeypox but the UKHSA has said a smallpox vaccine does offer some protection.
First identified in monkeys, the viral disease typically spreads through close contact and largely occurs in west and central Africa. It has rarely spread elsewhere, so this fresh spate of cases outside the continent has triggered concern.
The UKHSA has said a notable proportion of recent cases in Britain and Europe have been found in gay and bisexual men.
Reporting by Kylie MacLellan. Editing by Andrew MacAskill and Louise Heavens