School uniforms must be double-checked before being handed to students in Shanghai following an incident last month where they were found to contain cancer-causing dyes.

Before leaving the factory, uniforms must be sent to authorized centers for testing, while schools must send samples for testing and only hand uniforms to students when the quality is guaranteed.

This is according to a statement released by Shanghai's education, quality supervision and other departments.

The quality supervision department reported that uniforms produced by the Shanghai Ouxia Clothing Company contained carcinogenic aromatic amine dyes. Students from 21 schools in the city were ordered to stop wearing the uniforms.

Authorities said they will strengthen supervision of school uniforms.