Scientists determined that the parrot, imported from South America, died of the strain of avian flu that has devastated poultry stocks and killed 61 people in Asia the past two years, according to the Department for Environment, Food and Rural Affairs.
The virus is spread by migrating wild birds and has recently been found in birds in Russia, Turkey and Romania, spurring efforts around the globe to contain its spread.
While H5N1 is easily transmitted between birds, it is hard for humans to contract. But experts fear it could mutate into a form of flu that is easily transmitted between humans and cause a pandemic that could kill millions.
Debby Reynolds, DEFRA's chief veterinarian, said the parrot was likely infected with the virus while it was housed in the country's quarantine system with birds from Taiwan. Tests conducted on the Taiwanese birds that had died were inconclusive, according to the department.
DEFRA said the virus was most closely matched to a strain found in ducks in China earlier this year but was not very similar to strains discovered in Romania and Turkey. The genetic makeup of the virus changes slightly as it spreads, and scientists use such tests to track its migration across the world.
It was Britain's first confirmed case of bird flu since 1992.
Elsewhere, the Croatian government on Sunday promised to compensate villagers and farmers whose birds were slaughtered to prevent the spread of bird flu. About 10,000 domestic birds have been killed in an area near a national park where six swans were found to have been infected with the virus.
Damage from the culling was estimated at about $160,000. However, international bans on Croatian poultry exports could hurt farmers more. The European Commission on Friday said it was preparing a ban on all poultry imports from the country, while some individual European nations have already done so.
Medical experts detected the H5 virus in the swans Friday. Samples from the contaminated birds were then sent to a laboratory in Britain to establish whether they had the deadly H5N1 strain. Tests were also being done on samples from five other swans found dead Saturday morning near the park.