'I now ask my customers if they are happy to have prawns instead of oysters,' says Mr Li, 50, who sells oyster omelette, 'I only have two-week supply left, if I run out, I will have to close the shop earlier.'

Last year, only 21 per cent of molluscs passed the Agri-Food & Veterinary Authority (AVA)'s stringent health and safety requirements, compared to 93 per cent in 2006.

Currently, China is the major exporter of frozen raw oysters.

The 139.5 tonnes of frozen oysters were rejected because they contained novoviruses, which cause gastrointestinal illnesses and are spread by eating food contaminated with faeces from an infected person, or from polluted waters.

The virus has been linked to food poisoning outbreaks in December 2003 to January 2004 in Singapore.

'The viruses started appearing in the molluscs farmed in China since 2004.' said the AVA spokesperson.

Hawkers have been relying on their frozen stock, but as more consignments were rejected, importers' stocks have dried up and the trickle-down effect is being felt at the hawker centres.

Supplies would last for at most a week or two, according to the six hawkers in food centres at Newton, Bedok and Old Airport Road that The Straits Times spoke to.

Mr Chen Heng Xin, 46, hawker, has had no choice but to pay for pricier imports from Japan, which costs nearly 67 per cent more than those from China.

Suppliers like Mr Lee Tak Seng, an importer from TS Frozen Foods, says that this is the worst shortage in the 10 years he has been in the trade.

'I had to raise prices. I used to charge $7 to $8 per kg, but it is now about $14,' laments Mr Lee, 'I also had to dump 10,000 kg of goods because they were contaminated.'

TS Frozen Foods is one of the two main importers of frozen oysters.

AVA is currently assessing the pasteurisation process to deactivate the novovirus - that is, to heat the molluscs to a certain temperature which will kill the viruses.