Students at 21 primary and secondary schools in Shanghai have been warned to stop wearing their uniforms after tests showed they could be toxic.

The city's quality watchdog recently examined 22 batches of uniforms, including tops and trousers, and discovered some of six batches contained toxic substances.

Items produced by Shanghai Ouxia Clothing were found with aromatic amine dye, which can cause cancer, the authority said.

Authorities in Pudong New Area released a list of 21 schools that had purchased such uniforms from the factory.

Ouxia Clothing is a small manufacturer and has been producing school uniforms for five years, selling about 15,000 annually.

The company said toxic substances were found in 50 samples that were not sold to schools. The samples have now been destroyed, the company source said.

Xu Hui, a worker at the company, said the problematic materials were purchased from nearby Zhejiang province and were sewed on uniform sleeves as decorations.

Items produced by the company have been on the quality blacklist for the past three years due to unqualified instructions or a high pH index, which may lead to skin allergies.

"Clothes made by (Ouxia Clothing) have serious quality problems and we will issue an administrative penalty," said Zhou Yitian, an official at Shanghai Municipal Quality and Technical Supervision Bureau.

The bureau said it had ordered the company to halt production and carried out a thorough investigation.

In China, schools purchase uniforms from factories under a guiding price by government, usually no more than 150 yuan ($24) for primary and middle schools, and 250 yuan for high schools.

Industry insiders said although there are no barriers to producing school uniforms, many large clothes manufacturers do not want to accept orders from schools due to the low profits and high labor costs.

About 90 percent of school uniforms are produced by small factories such as Ouxia Clothing, and more than half of the clothes produced by these workshops are unqualified, Shanghai Media Group TV reported.

"Although the case is in Pudong, I'm still not sure whether the uniform my son wears is safe," said Zhang Yuerong, mother of an 11-year-old student.

The Shanghai government said on Monday that the city will establish strict entry requirements for manufacturers producing school uniforms. Meanwhile, it will set up a blacklist of unqualified manufacturers.

In addition, it is considering adopting a centralized purchase model whereby the whole process of ordering and purchasing will be under supervision by a quality watchdog and students' parents.