The UK's Health Security Agency said the case was detected as part of normal national influenza surveillance and that the source of the infection was unknown.

Mira Chande, director of accidents at the UK Health Security Agency, said: "This is the first time we have detected this virus in humans, as this strain is very similar to the viruses detected in pigs.

British authorities are following up on close contacts of the case, a statement said, adding that the situation was under intensive surveillance in operating theatres and hospitals in North Yorkshire, northern England.

In 2009, the swine flu pandemic infected millions of people and the cause of the flu was a virus containing genetic material from viruses that were circulating among pigs, birds and humans.

The UK's Health Security Agency said that based on preliminary information, the strain causing the infection detected in Britain differs from the strain detected in about 50 other human cases or so of the strain that has been detected elsewhere globally since 2005.