The programme, made after a four-month investigation, focuses on Harvest Rich and Evince, suppliers based in Dhaka, the capital of Bangladesh. Both are established suppliers to Tesco and produce clothes under the Florence and Fred brand, one of the supermarket group’s clothing lines. They also supply other Western companies.

The programme features footage of children who are allegedly under 15. In one interview, a 12-year-old who claimed to work as a machinist at Harvest Rich, says: “They (the factory) did ask me about my age. I said I was 11 years old. And they took me in.” He also claims that “around 200-300 child workers” are employed by Harvest Rich. In another interview a 12-year-old girl, who claims to work for Harvest Rich said she earns £9 a month.

Under Bangladeshi law it is illegal for children under the age of 14 to work.

The allegations are potentially embarrassing for Tesco, which has prided itself on its strict policy on ethical trading. The supermarket giant is one of the founding members of Ethical Trading Initiative, an organisation that promotes codes of practice governing working conditions for suppliers making products which will be sold in the UK.

Under the ETI code, suppliers are banned from employing children under the age of 15. They must also ensure staff are paid “living wages” and do not work excessively long hours.

Tesco said as soon it was alerted to the allegations more than a week ago it carried out “unannounced inspections” of the four sites featured in the report but found “no evidence whatsoever of under-age workers”. It added that all factories authorised to produce Tesco clothing in Bangladesh had been inspected by Tesco and independent specialists in the last year.

Tesco, which sources clothing from more than 30 countries, added: “Our inspections are detailed and thorough, involving a combination of record checking and on-site interviews with factory workers and we remain confident that they ensure all factories comply with the high ethical standards expected by our customers.”

The company, which last week announced record first-half profits of £1.09 billion, is now Britain’s third-biggest seller of clothing.

Channel 4 News’s full investigation can be seen in Monday’s programme at 7pm.

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