RSPB Cymru have confirmed that a highly pathogenic avian influenza (HPAI) has reached Grassholm Island off the Pembrokeshire coast after a spate of gannet deaths.
The island is home to the world's third largest colony of the bird species, with 36,000 pairs. It is also one of only two gannet colonies in Wales.
In recent months, HPAI cases have been identified in other parts of England and Wales.
The avian charity said it has been "living in hope" that Grassholm would manage to avoid the spread of the disease.
However testing by the Department for Environment Food and Rural Affairs (DEFRA) has confirmed the bird population on Grassholm has been affected by the influenza strain, following a spate of suspicious gannet deaths.
The outbreak on the island is currently small, with the number of birds affected low.
RSPB Cymru warned the situation could escalate so it will be closely monitoring the island. It added that it will update the public "as and when there is more to tell".
The island is not open to the public, as is normally the case.
Gannets are one of our largest seabirds and are white in colour, with black wingtips and yellow feathers on their heads.
They breed in large numbers but only at a few localities and are considered an Amber list species - meaning their conservation is of moderate concern.
If you find a dead or ill gannet or seabird on the beaches around Pembrokeshire, people are being advised not to touch them, keep dogs away from them and report the encounter to DEFRA.