Influenza A(H1N2)v has been found in a human in the UK for the first time.
Swine flu has been circulating in pigs in the country, the UK Health Security Agency (UKHSA) confirmed.
The case was detected as part of the routine national flu surveillance taken by the UKHSA.
It was discovered after the person affected was tested by their GP after experiencing respiratory symptoms.
One patient had the case of swine flu
The UKHSA said: "The case was detected as part of routine national flu surveillance undertaken by UKHSA and the Royal College of General Practitioners (RCGP).
"The individual was tested by their GP after experiencing respiratory symptoms. Influenza A(H1N2)v virus was detected by UKHSA using polymerase chain reaction (PCR) testing and characterised using genome sequencing."
The patient experienced mild symptoms and has now recovered.
The source of their infection is not yet known and an investigation is currently underway.
H1N1, H1N2 and H3N2 are subtypes of swine influenza viruses that are often found in pigs.
Those who are experiencing respiratory symptoms are urged to follow current guidance, which includes avoiding contact with others.
They can occasionally affect humans and this will often happen following direct or indirect exposure to pigs or to a contaminated environment, UKHSA explained.
Incident director at UKHSA Meera Chand said: "It is thanks to routine flu surveillance and genome sequencing that we have been able to detect this virus.
"This is the first time we have detected this virus in humans in the UK, though it is very similar to viruses that have been detected in pigs.
Those who have respiratory symptoms should stay clear of others
"We are working rapidly to trace close contacts and reduce any potential spread.
"In accordance with established protocols, investigations are underway to learn how the individual acquired the infection and to assess whether there are any further associated cases."
Chief veterinary officer Christine Middlemiss added: "We know that some diseases of animals can be transferred to humans – which is why high standards of animal health, welfare and biosecurity are so important.
"Through our animal and human surveillance systems, we work together to protect everyone.
"In this case we are providing specialist veterinary and scientific knowledge to support the UKHSA investigation. Pig keepers must also report any suspicion of swine flu in their herds to their local vet immediately."