The investigation highlights the potential problems manufacturers wanting to sell their products in China face if unlicensed ingredients are used. China recently announced that it was cracking down on food safety and targeting unlicensed ingredients is one method being deployed.

"We will find out whether the two firms have used GMO ingredients that are not approved by the ministry in their food products, and will deal with the issue based on the results," said Zou Ping, deputy director of the GMO security office with the MOA.

Greenpeace said last week that independent tests detected GMO ingredients in four snacks made by Ezaki Glico, a Japanese firm, and one by Germany's Metro Group, as part of a check on 21 samples of products sold in supermarkets in Beijing, Shanghai and Guangzhou.

GMO agricultural products must be MOA licensed and have identification labels on packaging before it can be sold on the Chinese market, according to Zou.

However, the laws are not applicable in China to biscuits, cookies, which use agricultural products as ingredients.

Greenpeace accused both firms of "double standards" in supplying Chinese consumers with food containing GMO, while both have promised to stop using GM ingredients in the products sold in their own countries.

"Like Japanese and German consumers, Chinese consumers do not want genetically engineered food. Glico and Metro should stop having double standards and apply their non-GE policy to China as well," said Greenpeace campaigner Lorena Luo.

A Greenpeace survey published earlier this month found that most Chinese consumers in large cities were against GMO food, although it also found many were unfamiliar with it.

Consumers in Beijing, Shanghai and Guangzhou were surveyed and overall 65 per cent said they would not choose GMO food and 77 per cent would not buy GMO rice.

Greenpeace is calling on the Chinese government to improve its labeling regulation to protect consumers' right to informed choice.