High-street clothes retailers Next and H&M found Syrian refugee children being employed in Turkish factories that supply them, according to a survey by a British charity released on Monday.
The report by the Business and Human Rights Resource Centre, a corporate responsibility watchdog, said both companies had acted to tackle the problem after finding Syrian children working in three of their suppliers.
But the charity warned that for other big brands "refugee workers appear out of sight and out of mind", saying only 10 of the 28 companies surveyed responded fully and five failed to do so at all.
The charity says an estimated 250,000 to 400,000 Syrian refugees are working illegally in Turkey, from around 2.2 million who have fled the almost five-year civil war at home.
It added that the Turkish government's move last month to issue work permits for Syrian refugees would reduce their vulnerability although it warned that "many refugee workers are likely to remain illegal".
The charity called on brands to communicate more with local trade unions and with non-governmental organisations that work with refugees.
"The treatment of Syrian refugees in their supply chains is a litmus test for high street brands' concern for human rights," said Phil Bloomer, the charity's executive director.
"Too many brands really didn't give us anything like the answers you would expect for a situation of such high-risk," Bloomer told AFP.
"And this is after all somewhere where the private sector can really make a major difference to the welfare of Syrian refugees but also to the flow of refugees, into Europe," he said.
Under the Turkish regulations, new arrivals will able to apply for a work permit six months after they obtain temporary protection status and those who have already resettled will be able to apply too.
Rights activists have long pressed Turkey to grant work permits to refugees -- most of them working illegally and for very low wages -- to allow them to build better lives in the country.
United Nations guidelines on child labour vary according to the type of job and how many hours they work. The Business and Human Rights Resource Centre survey asked brands whether they had identified child labourers, without asking about how old the workers were.