The tests on three ducks found dead in Romania's Danube delta near the Black Sea last week confirmed fears that the disease, which has killed more than 60 people in Asia since 2003, has entered Europe.
"We have received telephone confirmation from London that it is the H5N1 virus," Alina Monea, spokeswoman at Romania's veterinary and animal health authority, said on Saturday.
A European Commission spokesman in Brussels said: "We are waiting for the results which are supposed to be coming from Britain at about 1pm [on Saturday].
"I cannot confirm or deny [the report] but the commission was acting with the presumption that it would be this more dangerous type and took all the preventative measures," spokesman Robert Soltyk said.
Turkey also reported an outbreak of the deadly strain earlier this past week.
Experts fear H5N1 could mutate into a virus that spreads easily among humans, creating a pandemic that might kill tens of millions.
Romania has not reported any cases of bird flu in humans.
The Danube delta contains Europe's largest wetlands and is a major migratory area for wild birds coming from Russia, Scandinavia, Poland and Germany.
The birds mainly move to warmer areas in North Africa, including the Nile delta for winter.
Officials in Romania have announced plans to slaughter thousands of birds to prevent the disease from spreading.