Vaccination of all poultry flocks in Europe against avian flu should be considered in the wake of the outbreak of the deadly H5N1 virus on a turkey farm in Suffolk, a senior European public health expert said yesterday.

As the operation to gas almost 160,000 birds on the Bernard Matthews farm in Holton continued, Professor Koos Van der Velden, the chairman of the European Influenza Surveillance Scheme, in Utrecht, Netherlands, said the outbreak would increase the pressure on European governments to step up counter-measures.

The UK is expected to be at the top of the agenda at tomorrow's routine meeting of the EU's veterinary experts in Brussels. Thirteen EU countries have been hit by avian flu since the beginning of last year and the Standing Committee on the Food Chain and Animal Health - senior government vets from 27 countries - already require member states to ringfence outbreak sites and impose strict controls on the movement of poultry - as has been done in Suffolk.

Professor Van der Velden said: "There should definitely be a debate about vaccination [of poultry]. In Asia more and more countries are vaccinating. In the Netherlands we vaccinate. Its effectiveness is not proven but it should be discussed."

UK scientists fear vaccination could mask the start of an epidemic because it reduces the infectiousness of birds and stops them dying but does not halt the spread of disease. The Netherlands vaccinates 90 per cent of each flock, leaving 10 per cent unprotected. "When the virus strikes you can see its impact. That is a clever tactic," Professor Van der Velden said.

Supermarkets were bracing themselves for a sharp fall in poultry sales this week, fearing publicity about the outbreak would deter shoppers. Sales in Italy fell 40 per cent and in France by 20 per cent after similar outbreaks.

A spokeswoman for Sainsbury's said: "We have seen two scares before - one in Scotland and one in Norfolk - and we didn't see a dip in sales then. This time may be different but the facts are that you cannot get avian flu from eating poultry and eggs.

"We hope those facts have got across and consumers in the UK are wiser. The disease is not in the food chain, the people who have contracted avian flu have done so from working with ill poultry."

The mystifying aspect of the outbreak is how the virus penetrated a supposedly bio-secure poultry shed run by one of Britain's most successful poultry producers. Professor John Oxford, a virologist and flu expert from St Mary's College, London, said it had been expected that the first outbreak would occur on a small free-range farm where chickens had contact with wild birds.

A wild bird carrying the virus might have entered the Bernard Matthews shed via a ventilation shaft, he said. Other experts said droppings from an infected bird falling on the concrete apron outside the shed could have been walked into it on the shoes of workers.

A spokeswoman for the Department of Environment, Food and Rural Affairs said yesterday there were no new reports of infected birds and the investigation into what triggered the outbreak was continuing.

The UK held 10 million doses of vaccine to be used to create a buffer zone should further outbreaks occur, but had no plans to introduce routine vaccination.

Patricia Hewitt, the Secretary of State for Health, said yesterday the Government was preparing "very, very seriously and thoroughly" for a pandemic triggered by avian flu.

Although there is a low risk of humans being infected directly with avian flu, scientists fear it could mix with a human flu and mutate to create a new pandemic strain. Ms Hewitt added: "It is a very remote risk."

David Nabarro, who heads the UN department co-ordinating the global fight against the virus, said the world should expect more avian flu outbreaks in the coming months.