Exports from a Guangzhou poultry farm have been suspended after the bird flu virus was found in a nearby market.
The Centre for Food Safety said it learned from the Guangdong Entry-Exit Inspection and Quarantine Bureau that a H7 subtype of the virus was found in a poultry sample taken from a market stall in Zengcheng on Wednesday.
Poultry from the registered farm, located within a 13-kilometre radius of the market, tested negative for the virus.
But the Guangzhou health authorities temporarily suspended exports of the farm's live poultry and other products - including frozen poultry and eggs - to Hong Kong.
Currently, all live poultry from the mainland have to come from registered farms and pass the test for bird flu before being exported.
Since April 11, samples of the mainland's live poultry also undergo H7 tests at the Man Kam To Animal Inspection Station when they enter Hong Kong. None of the samples so far have tested positive for the virus.
If the sample collected from the market in Licheng town, Zengcheng, tests positive for H7N9, it would be the second discovery of the fatal strain of bird flu in Guangdong.
Last month in the province's Dongguan city - about 90 kilometres from Hong Kong - a sample out of 130 collected from poultry in a wholesale market tested positive for the H7 subtype of the virus.
But no human infection of the virus has so far been found in Guangdong province.
Former government veterinary officer Howard Wong Kai-hay said all live poultry from the mainland should be banned for 21 days to give time for tests on other registered farms.