UK public health officials on Monday said they had confirmed a first human case of a swine flu strain similar to one that has been circulating in pigs.
The variant of the H1N2 virus was confirmed in an individual who had been tested by their doctor after experiencing respiratory symptoms.
It has not previously been detected in humans in the country, the UK Health Security Agency (UKHSA) said.
"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," said the agency's incident director Meera Chand.
"We are working rapidly to trace close contacts and reduce any potential spread."
The individual concerned experienced a mild illness and had fully recovered, the agency said in a statement.
The source of their infection, however, was undetermined and is being investigated.
UKHSA chief veterinary officer Christine Middlemiss said veterinary and scientific knowledge is being provided to support its probe.
Influenza A(H1) viruses are endemic in swine populations in most regions of the world.
The H1N1, H1N2 and H3N2 viruses are major subtypes of swine influenza A viruses in pigs.
They occasionally infect humans, usually after direct or indirect exposure to pigs or contaminated environments.
The H1N1 pandemic in 2009 was the first major influenza outbreak in the 21st century.
The official death toll of 18,500 was later revised upwards by The Lancet medical journal to between 151,700 and 575,400 dead.