"Even the Centre for Food Safety does not know how much genetically modified food is on the market and how much we are consuming."

The centre currently tests thousands of food samples each month for pathogens, heavy metals, pesticides and the like, but they are not inspected or marked for GM content.

"It's not hard. All they have to do is include in the analysis whether the product is GM and disclose what gene was modified," said Chang.

"The government can then keep a public record of what GM foods are present in the market and then allow consumers to choose themselves."

Concern over GM products was raised after Secretary for Food and Health Dr Ko Wing-man last week said the government did not know if GM rice containing the potentially dangerous BT63 rice from the mainland had been imported into Hong Kong. His comment came after reports that the rice was being sold in Guangdong and Fujian provinces.

Though the rice is certified biologically safe, it has not been approved for cultivation or commercial purposes.

It is illegal to commercially grow or sell GM rice on the mainland but there are no laws to regulate the sale of any GM products in the city. The Hong Kong government also keeps no data on what foods have GM content.

Genetically modified foods can theoretically be sold in the city and consumed without the customer's knowledge.

Consumer Council chief executive Gilly Wong Fung-han agreed that improved testing for GM foods was required.

"As there is no regulation requiring traders to label the GM content in food products, no information is available on the prevalence of imported food products containing GM content," she said.

"It would be good to have regular mandatory testing for GM on imported food by the CFS to detect the presence of non-authorised GM crops or ingredients."

She also urged mandatory labelling as it would ensure consumers were able to make an informed choice on whether or not to buy GM foods.

Wong added that most assessments of GM products on the international market were thorough and had not indicated any risk to human health.

A Centre for Food Safety spokeswoman said if food samples were proven safe in tests, they would be considered safe for consumption - regardless of whether they were genetically modified. Identifying the contents would only help consumers make an informed choice.

The centre said a voluntary labelling scheme has been in place since 2006 and the industry was encouraged to follow the guidelines.

But the scheme has proved less than effective.

A test by the Consumer Council last year found 12 out of 49 samples of prepackaged food contained GM corn ingredients. Five of the samples exceeded the 5 per cent threshold, and none were labelled GM.

Food industry insiders are sceptical about whether tests or labelling would have much effect. "We've been talking about this for about 10 years now," said Simon Wong Ka-wo, chairman of the Chamber of Food & Beverage Industry of Hong Kong. "But these things would be difficult to enforce, logistically."

Wong argues it is more important that the public is educated on the issue, thereby encouraging companies to avoid GM products for fear of losing consumers. "The problem is, Hong Kong isn't a food manufacturing country - we have to mostly rely on other countries to ensure that the food is safe," he said.

Dr Thomas Lo Wai-hung, an associate professor at Polytechnic University's applied biology and chemical technology department, said the biggest problem was not GM food per se, but unapproved GM food imports, from the mainland for instance, coming into Hong Kong.

No standard policy for rules on GM food

The regulations for genetically modified food products vary widely across the globe.

In the United States - the world's largest commercial grower of GM crops - GM products have to be assessed by an agency within its Department of Agriculture, but its Food and Drug Administration does not require the goods to be approved before they are marketed for sale.

US companies - like those in Hong Kong and Canada - are not obliged to label GM produce.

But in the EU, all products that contain more than 0.9 per cent genetically modified organisms have to be labelled accordingly. EU regulators approve a GM food product for sale only if scientific research has proved beyond doubt that it is safe.

While no reports of ill-effects from eating GM food have been documented, its opponents argue the risks to public health have not been adequately identified. They also fear contamination of the conventional food supply.

Singapore, like Hong Kong, relies heavily on outsourced produce. It imports 90 per cent of its food from around the world.

The city state's genetic modification advisory committee admits this means much of the food products are "very likely" to contain "GM-derived components".

Food safety in Singapore is regulated by the Agri-Food and Veterinary Authority, which has approved the use of GM canola, cotton, maize, soybean and sugar beet. The city state has no guidelines on labelling GM products.

Its genetic modification advisory committee argues that countries that do enforce labelling do so more to empower consumers to make their own choices rather than on the grounds of food safety.

Assertions along these lines have been echoed in Hong Kong, where the government uses the fact that most of the city's food is imported as a reason not to enforce both labelling and testing.

Simon Wong Ka-wo, chairman of the city's Chamber of Food and Beverage Industry, said it was possible that demand for GM food products would gradually decrease and they might soon become a thing of the past.

"Many countries have abandoned GM products, so in the long run, they will disappear from the market altogether because no one will buy them any more," Wong said.

But Polytechnic University biologist Dr Thomas Lo Wai-hung said: "Where or not GM products are harmful is still heavily contested among scientists.

"Without conclusive evidence that such products are safe, consumers should be allowed to know what has been genetically modified and what has not."