The patient developed flu-like symptoms on October 21 and a rash three days later.
They are being treated at the Royal Free Hospital’s specialist high consequence infectious diseases unit in London.
In Africa, the Democratic Republic of Congo, Kenya, Burundi and Rwanda have all reported cases of Clade 1b mpox.
The patient’s close contacts, which includes housemates, are being contact-traced. This is thought to be fewer than 10 people.
“This is the first time we have detected this Clade of mpox in the UK, though other cases have been confirmed abroad,” said Prof Susan Hopkins, the chief medical adviser at UKHSA.
She said: “The risk to the UK population remains low, and we are working rapidly to trace close contacts and reduce the risk of any potential spread.”