Mass vaccination of British poultry in the event of a bird flu outbreak has not been ruled out, Environment Secretary Margaret Beckett has said.
Poultry are being vaccinated in France after H5N1 was found
But she said questions remained about how effective it would be in preventing the spread of the H5N1 virus.
Her comments came as the vaccination of nearly a million free-range ducks and geese began in south-west France.
In the UK, people with 50 or more poultry have until 28 February to register with the government.
Mrs Beckett said there were reservations as to whether vaccination would be appropriate.
Each bird must be vaccinated twice with a three-week gap, she said.
Speaking at the National Farmers' Union conference, she said: "As I expect you know, there is a question about how effective it could be - whether it is likely to mask the disease and decrease its spread rather than prevent it.
"We don't rule it out, we do keep it under review."
She said keeping poultry indoors would not "absolutely prevent the spread of diseases" but does offer "greater protection" if the virus affects wild birds.
Poultry keepers have expressed concerns to the NFU, which says it was keeping the avian flu contingency plan under review.
NFU president Tim Bennett said: "Certainly my poultry keepers are concerned about the fact that it is now in France and they are asking me if there is any more we can do in terms of bio-security and vaccination.
"If the scientists recommend vaccination, we will go with vaccination."
Mr Bennett said the industry was also worried about consumer confidence.
"If our consumers lose confidence in what is happening, that is what will finish the poultry industry, not avian influenza."
He praised the industry for registering on the new database, established by Defra in December.
"It is important to know where the birds are. If you have an area that is at risk and you know where the poultry are in that area, it is obviously going to be easier to get a bio-security plan in place," Mr Bennett said.
Meanwhile, the government's chief scientific adviser Professor Sir David King warned that with bird flu detected in France, the chance of it spreading to the UK had risen.
However, he was "optimistic" that a potential bird flu outbreak would not be as bad as the foot-and-mouth outbreak of 2001.
"If one poultry holder should go down, that in itself doesn't mean that other poultry holders will go down as well because the spread of the disease is going to be very different. It will not be the same spread process," he told the NFU.
"I feel - and I am prepared to stand here and say this - rather more optimistic about a potential H5N1 outbreak in poultry-holders in the UK than I would have been about foot-and-mouth back in 2001."