Poultry on an East Anglia farm close to the site of the bird flu outbreak are to be slaughtered on suspicion of carrying the disease, it was announced today.

Some 5,500 turkeys at Grove Farm in Botesdale, Suffolk, had already been earmarked for a precautionary cull as the premises shares staff with the infected Redgrave Park Farm.

But the cull has become one of "slaughter of suspicion" after animal health officials tasked with killing the birds arrived at the farm to find several already dead, the the Department of Environment, Food and Rural Affairs said.

An urgent inquiry is under way to identify the source of the latest outbreak. Today the investigation was focusing on the possibility that the virus could have been carried over by Dutch bird imports, though government vets have also given warning that it may already be endemic in the British wild bird population.

Yesterday, Defra officials announced that 22,000 free-range turkeys at Grove Farm and three other sites were to be culled as a precautionary measure due to "dangerous contacts" with Redgrave Park Farm, where the highly pathogenic H5N1 strain of avian flu had been detected.

The four premises are within the existing 10km (6 mile) surveillance zone, while Grove Farm is within the 3km (1.9 mile) protection zone. All five farms are operated by Redgrave Poultry and share staff due to their close proximity and relatively small scale of the operations.

The virus has not been confirmed on any of the three other sites, which have been named as Stone House Farm in West Harling, Bridge Farm in Pulham, Grove Farm in Botesale and Hill Meadow in Knettishall.

Around 5,000 turkeys, 1,000 ducks and 500 geese have already been culled at Redgrave Park Farm, near Diss in Norfolk, after the disease killed at least 60 turkeys.

News of the outbreak prompted Japan today to suspend all poultry imports from Britain, one of its largest suppliers of live chicks.

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