Shanghai authorities have launched an investigation into the sources of school uniforms found to contain cancer-causing dyes.

The disciplinary inspection department of Shanghai Pudong Education Bureau said on Tuesday it has started to investigate the case, including the purchasing and sales of toxic clothing materials.

According to the city's quality watchdog, the uniforms, which contain carcinogenic aromatic amine dyes, came from the Oulin Textile Co Ltd in Shaoxing, Zhejiang province.

Shanghai's quality supervision department said it is working on the investigation with its counterpart in Zhejiang.

Earlier, the Shanghai department reported that school uniforms produced by the Shanghai Ouxia Clothes Company contained carcinogenic aromatic amine dyes. Students from 21 schools in Shanghai have been ordered to stop wearing the uniforms.

Officials said the case has revealed loopholes in supervisory work on school uniforms, and an absence of clear job responsibilities in government departments.