For the first time in British waters, two dolphins have died of bird flu, the government has announced.
The marine mammals were found in several locations last month, on beaches in Devon and in Pembrokeshire.
Both dolphins were found to be infected with the highly contagious H5N1 variant of avian flu, which has spread around the world over the past 18 months.
In the latest outbreak, millions of birds have died from avian flu, either from the virus itself or from its culling, but scientists are concerned that it may be spreading to mammals.
The virus has now been confirmed in 23 mammals in the UK. Bird flu has been detected in dolphins in other parts of the world, but these are the first cases in the UK. It has previously been found in foxes, otters and gray seals around the British Isles.
There is no confirmation so far that the virus can spread between mammals in the wild, and most wildlife that contract bird flu is believed to have contracted it by capturing infected birds.
However, there are concerns that H5N1 could potentially spread between mammals following the mass die-off of seals and sea lions elsewhere in the world.
The findings have been forwarded to the World Organization for Animal Health.
The risk to humans from bird flu is classified as very low and the only cases have been in humans who came into close contact with infected birds. There is no evidence of human-to-human transmission.
A spokesman for the Animal and Plant Health Agency said: “Samples taken as part of routine wildlife surveillance have shown the presence of bird flu in two dolphins and one harbor porpoise.
“The animals were found dead and it is very likely that they had predated infected wild birds.
“The presence of influenza of avian origin in mammals is not new, although it is uncommon, and the risk from the H5N1 strain to non-avian wildlife in the UK remains low.”
The government says there is no evidence of an increased risk to non-avian wildlife following the dolphins’ deaths.
People are advised not to touch sick or dead wild animals or birds and to wash hands thoroughly with soap after contact with an animal.