All birds caged in the affected stall turned out to be healthy but were slaughtered as a precaution, Dr Thomas Sit Hon-chung, assistant director for inspection and quarantine for the Agriculture, Fisheries and Conservation Department, said.

'The exact source of the infection is still unknown to us, but we suspect it is more likely to belong to some wild birds. Risk of human infection is still low,' Sit said.

'We believe it is also necessary to close down the garden to undertake a thorough cleansing. It will not reopen until all samples we have taken are free of the virus.'

An infectious diseases expert suggested the swab was more likely to have come from the pet bird, as it was unlikely a wild bird would get into a cage. But University of Hong Kong microbiologist Ho Pak-leung added there was little risk to human health.

'The risk from infected live poultry such as chickens, ducks or geese in wet markets is higher than from pet or wild birds,' he said.

Ho said Hong Kong was still using a first-generation vaccine on imported birds, whereas the strain of the H5N1 virus commonly found in the city was of the fourth generation.

'It is time for the government to review whether the Dutch vaccine we have been using for years is still protective enough for the virus,' Ho said.

Bird flu has been detected in 22 samples taken from birds this year - mostly from the New Territories - more than double the number collected in the previous two years from some 10,000 samples taken annually.

Yesterday afternoon, health officials disinfected the Bird Garden area and moved birds onto a truck; 25 bird stall owners and workers, as well as 14 agriculture department workers, were put under medical surveillance. None showed any symptoms.

There are 18 stalls in the garden in Mong Kok, with more than 10,000 birds for sale. Most are imported from Malaysia, with some from Taiwan and the Philippines.

Some stall owners said the government had overreacted. Chan Siu-ho, 60, who earns HK$20,000 to HK$30,000 a month selling birds and bird food, said he expected business to be affected for the next six months.

'People may avoid coming here, fearing the virus, even after the garden reopens,' he said. 'It's a total overreaction from the government. It's pretty normal that birds get sick.'

Au Sing-cheong, who keeps more than 100 birds worth up to HK$200,000, said the area had been very clean, and he suspected the infected droppings were from a wild bird. The garden was last closed during the bird flu outbreak of 2007.

Last December, around 20,000 chickens and other poultry were destroyed following a bird flu scare in Cheung Sha Wan.