It was found to contain the H5 virus on Friday but further tests last night confirmed it carried the virulent H5N1 strain, which can be lethal to humans.
Britain’s chief vet Dr Debby Reynolds said the H5N1 strain had never been seen before, but most closely matched the disease found in ducks in China earlier this year.
A second parrot that died in quarantine was also tested at the Veterinary Laboratories Agency in Weybridge, Surrey, over the weekend.
But tissue from the two birds was pooled into one sample, so it is not known if one or both parrots was carrying the deadly virus.
The birds were part of a mixed consignment of 148 parrots and soft bills that arrived from Surinam, South America, on September 16.
They were joined in the quarantine facility in Essex by 216 birds from Taiwan on September 27.
Dr Reynolds said it was likely the bird caught the disease in quarantine and added: “There are more tests under way on the birds from Taiwan because we have established that some of them died before October 16.”
It was not yet known how many Taiwanese birds had died before that date or how many are undergoing tests.
Dr Reynolds said the overall risk assessment had been reviewed yesterday: “In light of the recent cases in China, Russia, Kazakhstan, Mongolia, Turkey, and Romania, it concludes that there is a high risk of further global dispersion.”
Some 2,000 birds in eight quarantine facilities in Britain need to be checked.
Meanwhile, the Labour Party in Ireland is urging an EU ban on imports of all live wild birds.
Labour agriculture and food spokesperson Dr Mary Upton said it was serious for Ireland given the level of free movement between the two countries.
“I acknowledge the steps that the Irish Government has taken so far, but additional measures are required, including more effective checks at points of entry, particularly of those coming from areas where outbreaks of avian flu have been confirmed.”
Dr Upton said it was also absolutely reckless to continue to allow the importation of wild birds while the threat remains and an EU ban was absolutely essential.
“Every precaution possible must be taken to prevent it from reaching our shores,” she said.