The patient developed flu-like symptoms on 21 October and a rash three days later.
An mpox rash of pus-filled lesions can last for up to a month. Other symptoms include fever, headaches and low energy.
Laboratory testing confirmed it was Clade 1b. This form of the virus has been causing mounting concern.
Its close relative Clade 1a is largely connected to exposure to infected animals or eating bush meat.
But Clade 1b appears able to spread more easily from person to person through close physical contact, including sex.
The patient is 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 include housemates, are being 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 the UK Health Security Agency (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.”
Sweden, India and Germany have all detected cases of this strain of mpox linked to travel to affected countries.
This is a different outbreak to the one that primarily affected gay, bisexual and other men-who-have-sex-with-men in 2022, called Clade II.
Health and Social Care Secretary Wes Streeting said : “The government is working alongside UKHSA and the NHS to protect the public and prevent transmission.
“This includes securing vaccines and equipping healthcare professionals with the guidance and tools they need to respond to cases safely.”