Syrian refugee children have been working in factories in Turkey making clothes for British high street retailer Marks & Spencer and online store ASOS, an investigation by BBC Panorama found.

The investigation found Syrian refugees as young as 15 working long hours for little pay, making and ironing clothes to be shipped off to Britain.

BBC journalists took photographs of Marks & Spencer labels in the factories. Some Syrian refugees worked 12-hour days in a factory distressing jeans for fashion brands Mango and Zara, using chemicals with inadequate protection, the BBC said.

An M&S spokesperson said: "We had previously found no evidence of Syrian workers employed in factories that supply us, so we were very disappointed by these findings, which are extremely serious and are unacceptable to M&S."

An ASOS spokeswoman said: "It's a subject we take incredibly seriously. But it would be wrong for us to comment on reporting we haven't seen."

M&S said it was working with the Turkish supplier to offer permanent legal employment to any Syrian daily workers employed in the factory.

Turkey has been a main entrypoint for refugees from the ongoing conflict in Syria, with three million estimated to be living there. Ankara in March signed a deal with the EU to stem the flow of refugees into the bloc.

A Reuters investigation this year also found evidence of Syrian refugee children in Turkey working in clothes factories in illegal conditions.