More than 20 swans in the three countries have died from the H5N1 strain which experts say was probably brought by cold weather in the Balkans.
Laboratories in Padua, in Italy, and Weybridge, in Britain, made the identification yesterday.
H5N1 has killed birds in 20 countries and infected at least 166 people, killing 88 of them. The "majority" of the 17 dead swans found in southern Italy tested positive for the lethal strain and all three swans found in northern Greece were killed by it.
To limit the spread of the virus, Italy has banned the transport of live birds in six regions for 21 days, which is the incubation period of the virus, and are warning people about the disease.
Francesco Storace, the country's health minister, said that some of the dead birds had been killed by the high-pathogenic type of bird flu, meaning it kills almost all birds that are infected very quickly. "The good news is that these were wild animals," said Mr Storace at a press conference in Rome.
The Greek government said that it had discovered the H5 virus in a wild goose on the island of Skyros and was investigating to see if it was also killed by the H5N1 strain.
However, the country's ministers say they do not need to take additional safety measures as the country has been on bird flu alert for months after the disease spread to nearby countries such as Romania and Turkey.
The European Commission said the disease was last week detected in wild swans in the Bulgarian wetland region of Vidin, close to the Romanian border. Bird flu reached Romania in October.
The Bulgarian authorities also informed the commission of other suspected cases in wetlands close to the Black Sea.Reuse content