Doubts over testing in quarantine for bird flu have been raised after it emerged Taiwanese finches, not a parrot, brought the disease to the UK.
Samples from a parrot at the Essex centre were mixed with others (©www.parrotsandparakeets.com)
A government report said the mixing of tissue samples led officials to wrongly assume a South American blue-headed pionus was the source of the virus.
Opposition politicians said the report exposed confusion in the system and raised more questions than it answered.
But ministers argued it showed quarantine procedures were working.
The probe for the Department of Environment, Food and Rural Affairs (Defra) said it was more likely the virus was brought to an unnamed quarantine centre in Essex by 50 finches from Taiwan rather than by the parrot as previously thought.
Because the tissue samples of the first birds to die were mixed, it was unclear which birds had the H5N1 virus strain.
Later tests showed the Taiwanese birds were the "most likely" virus source, as H5N1 was not found in other species.
Brett Hammond, of Pegasus Birds, which imported the finches, said he did not think there was a problem with the existing way birds are tested.
He told BBC News: "There is strict regulation and the controls that are in force are there to protect the chicken population and obviously the general public to make sure there is no escape of any virus.
"I don't see there is any problem with regard to the current regulations although I believe they are going to make them even stricter than they already are."
But shadow environment secretary Oliver Letwin said the report had exposed confusion in the handling of the issue and that quarantine procedures should be tightened immediately.
Testing pooled samples for diseases may have been appropriate in the past but not now when the country was on a state of alert for bird flu, he said.
He added: "Defra informs us that it will be another three weeks before they will announce what they will do to strengthen the quarantine system. This delay is quite unacceptable."
His comments were backed by the former president of the British Veterinary Society Bob McCracken who said although pooling samples was standard practice in some cases, it should not have been done with two separate consignments.
There have been cases of the H5N1 strain detected in Europe
But he said this was a minor issue compared with the fact that the disease had been successfully contained in the quarantine centre.
Liberal Democrat environment spokesman Norman Baker said it was more important than ever the quarantine review produced recommendations that inspired public confidence.
He added: "This is turning into even more of a shambles than could have been imagined. This revelation raises more questions than it answers."
The report said the birds were grouped according to species and location in the quarantine facility and tested in line with accepted epidemiological sampling practice.
But chairman of the environment, food and rural affairs committee Michael Jack said if officials wanted to know where a disease had come from it would be sensible for tissue samples to be tested separately.
Environment minister Ben Bradshaw admitted the procedures for detecting the disease may have to be strengthened and said Environment Secretary Margaret Beckett had already set up a review.
But he said there was no cause for alarm as all the birds had been accounted for and none had been released into the country.
The report found the highly pathogenic strain of bird flu was initially identified when tissues from two birds that had died in the centre were analysed together.
These were a blue-headed pionus from Surinam and a mesia from Taiwan, which were found dead on 14 October.
The report concludes "on the balance of probabilities" the H5N1 was brought to the facility by the mesias.
This was because later tests showed that 53 Taiwanese mesias died from the disease, but that none of the birds from Surinam had.
Responding to the report, the Taiwanese government said: "There is a good possibility that profit-driven traders smuggled mesias from China to Taiwan, using our avian flu-free country as a front from which they laundered these birds to the UK and other countries."
A separate investigation is being undertaken by Essex County Council's trading standards department into the circumstances surrounding the quarantine of infected birds.