The council found the grey-blue trim on the blouse of the Lingnan school uniform contained 173mg/kg of the chemical – more than eight times what was allowed in mainland China. A checked waist belt for the Fung Kai school uniform contained 41 mg/kg, twice the permitted level.

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Wong Yuk-shan, chairman of the council, said when the azo dyes were mixed with students’ body heat and sweat, the harmful chemical could be released and absorbed by the skin.

“This is most unsettling and unsatisfactory,” he said.

The findings came after the council checked 49 samples of school uniforms – those with designs and colours most commonly used in the city – sourced from 22 suppliers. It found Zenith Uniform made the waist belt, while Sing Shun Fat School Clother Co made the blouse trim.

The city’s uniform shops usually have contracts with specific schools to make their uniforms, but there are no rules stopping them from making uniforms for other schools. After the council released its report, Lingnan Secondary posted a notice on its website saying Sing Shun Fat was not the official supplier.

“Parents who have bought uniforms from this company should immediately stop their children from wearing them and use only the school’s official supplier, Victoria Uniform,” the school said.

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Similarly, Fung Kai said its official supplier was Lam Chi Kee and told parents who used uniforms from Zenith to be aware of “potential health risks”.

Fung Kai said they had not received any reports from parents who had bought the belts from Zenith.

Zenith’s manager, who gave her name only as Ho, said the belt was made of cloth bought from a Sham Shui Po shop, which she claimed had already closed down. The company had only made about 20 uniforms about a decade ago and had sold some, one earlier this year. It had disposed of the rest.

“We used the cloth to make belts 10 years ago and no one detected the cancer-causing substances,” Ho said. “We did not even know about this until now.”

Ho said the checked cloth was not used for any other uniforms.

Sing Shun Fat’s owner, who gave his name only as Yiu, said the Lingnan school blouses were made at a factory in mainland China and he was ordered to stop selling them by Hong Kong’s Customs and Excise Department in June.

The department on Monday said it had seized 17 pieces of the school uniform from a supplier when it conducted a check. It did not give the name of the shop.

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Yiu said the last batch of blouses in his shop were made about five years ago, as it lost the bid to be Lingnan’s uniform supplier after that. Other uniforms he sold were made from cloth from India or Indonesia, he said.

The council also found eight uniforms it tested had traces of formaldehyde, which is used as an industrial preservative and can also cause cancer. But the amounts were lower than 75mg/kg – the maximum allowed under mainland Chinese regulations.

The council’s chief executive Gilly Wong Fung-han said a low concentration of formaldehyde would not pose any carcinogenic risk to students, but could cause itching. Tests showed that, as formaldehyde is water soluble, some of it could come off in the wash, and Wong urged people to wash new uniforms before wearing them.

Hong Kong has no regulations of its own on textile products. Wong called on the government to address this.