Meat firm Bernard Matthews was warned several times about hygiene lapses at its Suffolk turkey farm before bird flu broke out, an official report says.
Turkeys have been arriving at the plant in Suffolk
Inspectors saw gulls feeding on waste left in uncovered bins and buildings with holes big enough for rats, the environment department, Defra, said.
Its report concluded the most likely cause of the Holton outbreak two weeks ago was poultry imported from Hungary.
Bernard Matthews said it had followed all of Defra's biosecurity regulations.
The Defra paper, which looked into the sources of the outbreak, was released on the same day as the Food Standards Agency (FSA) delivered its report on the outbreak.
The FSA found no evidence that meat had entered the UK food chain from infected areas of Hungary.
It concluded that all food importing and processing activities carried out at the Holton site had complied with EU laws.
The agency said Bernard Matthews staff had been verbally warned about "deficiencies" a number of times but the problems had been sorted out each time.
The FSA's veterinary director, Alick Simmons, said there were "concerns about the disposal of animal by-products", which needed to be assessed by Defra.
In a statement, the firm said it welcomed both reports, saying they confirmed that its conduct had "always been legal".
It said the authorities had identified ways in which biosecurity could be enhanced and the company would "comply with any recommendations".
"Bernard Matthews will continue to work closely with the authorities involved and wants to remind consumers that the FSA has confirmed that properly cooked poultry is perfectly safe to eat."
The Defra investigation found that:
- Pest control workers noted last month that large numbers of gulls attracted to uncovered bins full of trimmings from turkey breasts
- This had also been a problem last year
- Gulls were observed carrying turkey waste away and roosting on the roof of the turkey houses 500 metres away
- There were holes in the houses that could have allowed birds or rodents in
- Polythene bags containing residue of liquid waste could have blown around the site
- Plastic-covered bales of wood shavings used as bedding were kept outside
Defra stressed that its investigation into the outbreak was still going on.
The firm could still be prosecuted if it is found to have breached laws on the disposal of animal by-products.
Firm 'in denial'
Defra said wild birds were unlikely to be the source of the outbreak - as H5N1 has not been found in such birds since August last year, and surveillance at the Holton plant failed to find any infected animals.
Fred Landeg, deputy chief vet, said: "We may never be able to conclusively pinpoint the original source of the virus."
The H5N1 strain found on the site is nearly identical to that in Hungary - where Bernard Matthews has a plant which regularly supplied meat to its UK operation.
The firm said its voluntary decision to halt the movement of meat to and from Hungary - introduced on 8 February - would remain in place "until further notice".
Shadow agriculture minister Jim Paice called for further studies into the way the virus was being controlled, saying the studies "raised important questions".
Liberal Democrat environment spokesman Chris Huhne accused the company of being "in denial" over events at its plant.
"Allowing wild birds to feed on raw poultry meat left in the open is highly irresponsible, as it could lead to widespread contagion," he said.
Following a cull and a disinfecting operation, the plant started to receive consignments of turkeys from other sites in the UK on Tuesday.
Some Bernard Matthews turkey products have already been cleared by the FSA to be released for sale.
Meanwhile, a ban on licensed bird gatherings - including races, sales and shows - has been lifted in non-restricted parts of England, but remains in place in the restricted zone across Suffolk and Norfolk.