East Anglia's £2bn poultry industry held its breath today about the full impact of the avian flu outbreak in Suffolk as the public was urged to be vigilant and report sightings of both unattended farmed flocks and dead wild birds.

East Anglia's £2bn poultry industry held its breath today about the full impact of the avian flu outbreak in Suffolk as the public was urged to be vigilant and report sightings of both unattended farmed flocks and dead wild birds.

With officials still baffled about the source of the outbreak of the H5N1 virus at the huge Bernard Matthews' farm at Holton, near Halesworth, environment secretary David Miliband denied there had been delays in tackling the disease and reassured consumers that food supplies were safe.

Meanwhile, every household in the 3km protection zone around the site was today sent a letter by Suffolk County Council urging extra vigilance and advising that pet cats be kept indoors and dogs on leads.

And with officials primed to answer calls right across the restricted area of north-east Suffolk and south-east Norfolk, people were asked to report flocks not under cover and any dead birds such as ducks, swans and waders at once.

Workers clad in white protective clothing were expected to complete the cull of almost 160,000 turkeys at the farm tonight, with the on-site processing factory expected to remain closed until later in the week.

Mr Miliband, speaking in the Commons after a meeting of the Cabinet's emergencies committee (Cobra) this morning, insisted that Defra's response to the outbreak of the virus which is potentially lethal to humans had been “rapid, well co-ordinated and appropriate.”

And Richard Bacon, MP for South Norfolk, warned that “we are in danger of talking ourselves into a crisis when one is not warranted”.

But it was unclear tonight what effect the avian flu outbreak was having on shoppers.

While the world's media continued to camp out at the farm's perimeter, questions about how the deadly Asian strain of the disease reached a shed full of poultry in a quiet corner of East Anglia remained unanswered.

Defra, the government agency leading efforts to contain and eradicate the virus, said it was still unclear what caused the infection.

The government's chief scientific adviser said that a wild bird was the most likely source of infection. Sir David King said attention was focussing on Hungary after the virus proved to be “100pc the same” as an outbreak there a few weeks ago.

“That could been that it has been spread directly by wild birds from that region, but it could mean it has spread some other way from Hungary, for example on someone's boots,” he said.

Bernard Matthews also owns Saga Foods, Hungary's largest poultry producer, which Sir David said “might be significant”, though the firm denies that there is a physical link between the two companies.

“If you are asking me what is most likely, I would suggest it is the low prevalence in wild birds, in which case we all need to be alert for the possibility of incursion into other farms,” added Sir David.

Suffolk and Norfolk Euro MP Richard Howitt said: “The Holton outbreak follows extremely shortly after geese in Hungary proved positive for this strain, we must act on a Europe-wide basis to investigate whether this outbreak is linked, and if so how it has been spread.”

A suggestion that migrating wildfowl had brought the disease to British shores from Eastern Europe was played down by experts in Norfolk today.

The Thetford-based British Trust for Ornithology said it was the wrong time of year for flocks of birds to be arriving from Eastern Europe.

A spokesman for Defra said there were no plans to lift or extend the 3km and 10km zones or the buffer zone around the farm.

“The existing restrictions are remaining in place. There is the 3km exclusion zone, the wider 10km zone and the restriction zone of 2,000 square km. They will remain in place for the time being. It is too early to say when they will be lifted,” he said.

Meanwhile, officials from Suffolk County Council are still busy doing farm gate checks offering advice to farmers and others within the 3km zone and patrolling for loose flocks or dead birds.

Farm workers have also been reassured that the chance of infection passing from live birds to humans was small to the point of being “infinitesimal”.