'Sparrows were just everywhere here. They flew in and out through very small holes and they ate the chicken's feed. We have tried every means but there was no way we could get rid of them,' Mr Wong said.

Coops built from metal plates and nets were supposed to separate the chickens from wild birds but there were small gaps between the plates.

The Food and Health Bureau set up two investigation groups four days after chickens at the Yuen Long farm were found to have the H5N1 virus on December 8.

Mr Wong said inspectors from the Agriculture, Fisheries and Conservation Department visited his farm almost weekly and knew the chickens and sparrows lived together.

But a department spokesman last night declined to confirm whether it was aware of the situation, saying only that preliminary findings would be presented later this week.

Ho Pak-leung, a microbiologist at the University of Hong Kong, said the farm's bio-security measures had failed. 'Good bio-security measures mean the live chickens should be completely segregated from other animals or birds,' he said.

Professor Ho said sparrows in both Hong Kong and on the mainland had been found with the H5 virus and they could be the source of the latest outbreak.

He also said the department should have acted to step up the farm's bio-security measures.

The experts also suggested Mr Wong cover a 2-metre-square hole in a roof, which is now covered by only a net, to prevent the excretions of migratory birds from dropping into the chicken coop.