China is investigating claims vegetable sellers are spraying cabbage with harmful formaldehyde to keep it fresh, an official said on Tuesday, in yet another food safety scare to hit the country.

Dozens of vegetable dealers in the eastern province of Shandong, a major vegetable supplier, are using the chemical to keep produce fresh on the way to market, media and Internet reports said this week.

Formaldehyde -- commonly used as a preservative for laboratory specimens and embalming -- can be fatal if ingested and is also a cancer-causing substance.

A local government official confirmed the practice and said authorities had started an investigation.

"We are investigating this matter," an official from Dongxia town -- where the practice was originally uncovered -- told AFP. He declined to give details.

The practice is widespread in Shandong and neighbouring Hebei province, especially in warmer months, the official Xinhua news agency has reported.

"It's a common practice to keep the cabbage fresh. Otherwise, the vegetables stacked tightly in the trucks would rot in two to three days," a farmer in Dongxia was quoted as saying.

Cabbage is a staple of the Chinese dining table, especially in the country's north.

Chinese media say formaldehyde is used on other products, such as seafood and mushrooms. Officials say some sellers do not use costly refrigerated trucks for vegetable transport.

China's government has repeatedly vowed to improve food safety as people grow increasingly alarmed about the quality of what they eat, but scandals still occur due to weak enforcement and unscrupulous business practices.

Milk was at the centre of one of China's biggest food safety scandals in 2008 when the industrial chemical melamine was found to have been illegally added to dairy products to give the appearance of higher protein content.

Last year, authorities arrested more than 30 people over the sale of cooking oil made from leftovers taken from gutters.

More recently, employees of a leading Chinese poultry company sold diseased ducks to consumers, while a major dairy producer sold milk with high levels of a cancer-causing toxin, caused by cows eating mouldy feed.