Syrian refugee children have been making clothes for Marks and Spencer, it has been claimed.

An undercover Panorama investigation of factories in Turkey allegedly found children had been working on clothes for Marks and Spencer and for the online retailer ASOS.

Panorama – Undercover: The Refugees Who Make Our Clothes, to be broadcast on Monday night, will also claim that one factory boasted of making clothes for Next while employing Turkish children and Syrian refugees.

The documentary also claimed to have discovered refugees working illegally on Zara and Mango jeans.

The BBC said it had been told by all of the brands concerned that they carefully monitor their supply chains in Turkey and that they do not tolerate the exploitation of refugees or children.

The Panorama documentary makers, however, claim to have found seven Syrians working in a Turkish factory supplying Marks and Spencer.

The BBC said the refugees often earned little more than a pound an hour, well below the Turkish minimum wage, and were employed through a middleman who paid them in cash on the street.

One of the refugees alleged to the documentary makers that they were poorly treated at the factory, saying: “If anything happens to a Syrian, they will throw him away like a piece of cloth.”

The BBC said the youngest worker was a 15-year-old boy who was working more than 12 hours a day ironing clothes before they were shipped to be sold in UK shops.

A Marks and Spencer spokeswoman told The Independent: “Ethical trading is fundamental to M&S. We are acutely aware of the complexity surrounding Syrian refugees in Turkey. We have a local team on the ground in Turkey who have visited all of our suppliers there. They have also run supplier workshops on the Syrian refugee crisis highlighting the change in labour law and how to legally employ Syrian workers.

“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. We are working closely with this supplier to take remedial action, including offering permanent legal employment, in support of any Syrian daily worker who has been employed in this factory.”

She added: “All of our suppliers are contractually required to comply with our global sourcing principles, which cover what we expect and require of them and their treatment of workers.

“We do not tolerate such breaches of these principles and we will do all we can to ensure that this does not happen again.”

The BBC said Panorama reporter Darragh MacIntyre spoke to dozens of Syrian workers who felt they were being exploited. Mr MacIntyre said: “They speak of pitiful wages and terrible working conditions. They know they are being exploited but they know they can do nothing about it.”

In one back-street workshop in Istanbul, the documentary team allegedly found several Syrian child workers. In the office workshop, the documentary team said, they discovered an ASOS sample.

The BBC said ASOS told Panorama it accepted that although its clothes were made in the workshop, it was not a facility that had been approved by the retailer.

The BBC said ASOS subsequently inspected and found 11 Syrian adults and three Syrian children under 16 at work.

ASOS told the BBC the children would be financially supported so they could return to school and the adult refugees would be paid a wage until they were found legal work.

An ASOS spokesperson told the BBC: “We have implemented these remediation programmes despite the fact that this factory has nothing to do with ASOS.”

An ASOS spokesperson said the retailer took issues around child and refugee labour very seriously but was unable to comment in detail to The Independent until after company representatives had seen the full Panorama documentary.

Panorama also claimed to have found Syrian refugees working 12-hour days in a factory that was distressing jeans for Mango and Zara.

The BBC said that although the refugees were involved in spraying hazardous chemicals to bleach the jeans, most of the workers didn’t even have a basic face mask.

Mango told the BBC that the factory was working as a subcontractor without its knowledge. The company also told the BBC that a subsequent inspection by Mango representatives didn’t find any Syrian workers and found “good conditions except for some personal safety measures”.

Zara’s parent company, Inditex, told the BBC its factory inspections are a “highly effective way of monitoring and improving conditions”. The BBC said Inditex had already found significant non-compliance in an audit in June and had given the factory until December to make the necessary improvements.

In one Istanbul factory, Panorama reported finding Syrian adults at work alongside Turkish children as young as ten.

The BBC said the owner boasted that he had been working for Next and showed the undercover team a set of Next pyjamas that he claimed the factory had helped produce.

Next told the BBC the pyjamas were actually made by another supplier and the pyjamas seen by Panorama may have been a sample. Next also told the BBC that samples circulate widely and that the presence of a sample in a factory does not mean it was made there.

Representatives of Mango, Inditex and Next were not immediately available to talk to The Independent.

Panorama - Undercover: The Refugees Who Make Our Clothes will be shown on BBC One at 8.30pm on Monday October 24