The four chief veterinary officers of the UK said the cases of avian influenza were detected on Thursday.
The risk of transmission to people remains low, they added.
“Following two confirmed cases of H5N8 avian influenza in the Netherlands we have raised the risk level for incursion to the UK from migratory birds to medium ahead of the winter migration season," the statement said.
“The risk of the disease being introduced to poultry farms in the UK remains low.
“We are monitoring the situation carefully and bird keepers should remain alert for any signs of disease, report suspected disease immediately and ensure they are maintaining good biosecurity on their premises.”
They added poultry keepers should make sure their birds' living area is clean, place feed and water in enclosed areas protected from wild animals and remove any spillages, clean their footwear before and after visits, and put fencing around any outdoor areas the birds can access.
The vets also advised avoiding mixing ducks and geese with other poultry species.
Wild birds migrating westward from mainland Europe during the winter period can spread the disease to poultry and other captive birds.
This strain of the virus can be very virulent in birds, but there are no recorded cases of it causing disease in humans.
The UK was previously declared free of avian flu in 2017 and has remained free of highly pathogenic avian influenza since then.
Additional reporting by PA