Workers in British factories producing items for a number of fashion brands are being paid less than half of the legal amount, according to a Channel 4 documentary.

The undercover film, which will air tonight (23 January) at 8pm and is part of the broadcaster's Dispatches series, found a number of textile firms making garments for high street retailers such as River Island, Boohoo, Missguided and New Look pay their staff between £3 ($3.7m €3.5m) and £3.50 per hour.

That is less than half the legal minimum wage, which currently stands at £7.20 per hour for workers over 25.

In the documentary, an undercover reporter is employed by Fashion Square, which manufactures clothes for River Island, but when he questions his employer over his wage, he is told he will not receive the legal rate.

"You won't get that [the minimum wage] here," the reporter is told.

"That's what I'm telling you. We don't get paid much for our clothes, and we need to compete with China and Bangladesh. They can get it cheap there. How will they get it made cheaper here? If we pay everyone £10 or £6 then we will make a loss."

River Island told Dispatches that Fashion Square, which paid the undercover reporter £3 for an hour's work, was removed from its approved factory list in February last year.

"Suppliers were informed not to use this factory for any further orders," the company said in a statement.

"Subcontracting without River Island's approval is a serious breach of our terms and conditions."

The same reporter was also employed by United Creations, where he received £3.25 for an hour's work, which included marking up zips on dresses for Missguided and packing a jacket for Boohoo. The latter, however, claimed it was unaware United Creation was among the companies carrying out work for one of its approved suppliers, adding it would "not tolerate suppliers paying less than the minimum wage".

Missguided said it took the allegations very seriously, stressing it demanded the highest standards of safety, working conditions and pay from its suppliers and subcontractors.

"We are committed to achieving the standards set by the Ethical Trading Initiative and conduct regular audits and spot-checks of our supply chain," the company added.

"We have begun an internal investigation [...] we will also ensure these matters are addressed urgently by the supplier in the best interests of the workers."

Meanwhile, Dispatches showed the same reporter being paid £3.50 in a factory producing garments for New Look. The clothing retailer, which has cut the number of UK suppliers from 104 to 19 over the last six years, said it was "extremely concerned" by the findings.