Shanghai's quality control watchdog has ordered four clothing manufacturers to stop supplying student uniforms in the city after authorities discovered they had produced uniforms made with fabric containing carcinogenic chemicals, officials said at a press conference Tuesday.

The Shanghai Municipal Bureau of Quality and Technical Supervision found toxic aromatic amine dyes in the fabric of five batches of student uniforms it tested ahead of the new school year, according to the press release that the bureau issued after the press conference.

The bureau ordered the manufacturers to cancel all of their orders from local schools and recall any student uniforms already delivered to ensure the problematic uniforms don't end up on students' bodies.

It has started investigating the three manufacturers based in the city, and has notified authorities in another province about the fourth company, which is located outside of Shanghai.

The discovery of the tainted student uniforms resulted from a new measure that requires both schools and manufacturers to send uniforms to the quality supervision bureau for testing. In the past, schools have not been required to send in uniforms for testing.

The city's education authority agreed to the measure after the quality supervision bureau found out in February that a garment company had produced student uniforms containing carcinogenic chemicals for a school in Pudong New Area, the bureau said.

Shanghai Television Station reported at the time that the manufacturer, Shanghai Ouxia, had sold uniforms to more than 30 schools.

Prior to that scandal, the quality control watchdog had inspected student uniforms from only 20 manufacturers because the city's education authorities did not provide it with a list of manufacturers that supplied city schools with uniforms, said Shen Weimin, vice director of the Shanghai Municipal Bureau of Quality and Technical Supervision.

The bureau usually discloses the names of manufacturers whose products fail quality tests, but it did not do so in this case, even though it did publish a list of companies that failed a random test for minor PH value problems on its website.

Late Tuesday, the Shanghai Municipal Education Commission published the names of five schools on its microblog that had ordered the problematic uniforms. The list included Shanghai High School, one of the city's most prominent high schools.

Shanghai High School could not be reached for comment Tuesday. Most schools in Shanghai are closed for summer vacation. Neither the education commission nor the quality supervision bureau have disclosed how many tainted uniforms were produced.

There are 68 garment companies producing student uniforms for city schools. As of August 20, a total of 50 companies and 178 schools have sent in uniform samples for testing.

The Shanghai Municipal Education Commission has started requiring all local schools to publish information about uniform procurement to make the process transparent for students and parents. It has also required schools to send copies of their procurement contracts to district education bureaus.

The commission publicized the measures it has taken to ensure the quality of student uniforms during a press conference Tuesday at a primary school in Pudong New Area.

The uniforms that Shanghai Zhuyuan Primary School has bought for the upcoming school year have all passed the quality supervision bureau's battery of  tests, said Xie Zheng, director of the school's logistics support office.

The school has also published the results of the school uniform quality tests, copies of procurement contracts, the selection criteria for those contracts and information about student uniform manufacturers in Pudong New Area.