Clothing on sale in the high street is being made in Britain in dirty, dangerous and “appalling” conditions, according to secret television footage which will intensify pressure on a fashion industry hit by a series of sweatshop scandals.

For three months, a reporter for Channel 4’s Dispatches worked undercover in workshops in Leicester, stitching garments for British retail chains including Bhs, run by Sir Philip Green, who is advising David Cameron on public sector “efficiency” savings.

In the investigation - to be screened tonight - the undercover reporter found:

* “dangerous, pressurised sweatshop conditions”;

* pay at half the legal minimum wage;

* workers exhorted to work faster under threat of the sack;

* cramped and over-heated conditions with unsanitary toilets and at least one blocked fire exit.

Workers’ identity and legality was also not checked, according to the programme, Fashion’s Dirty Secret.

The factories were making clothes for five high street brands: Bhs, owned by billionaire retailer Sir Philip’s Arcadia group, New Look, Peacocks, C&A and Jane Norman.

All of the companies, with the exception of Jane Norman, are members of the Ethical Trading Initiative, which has a code of conduct aimed at preventing abuses of workers, or they have their own supplier codes of conducts.

Several of the companies launched immediate investiations into the programme’s findings. Arcadia said the investigation showed that a supplier sub-contracting out the work had “clearly breached” its code of conduct.

C&A group described the abuses uncovered as “appalling”. In a statement, C&A said: “The working conditions described... constitute a blatant violation of international social standards, national laws and individual rights. We find such practices appalling.”

Undercover investigators have regularly found back-breakingly long hours, low pay and dangerous conditions at foreign sweatshops supplying British retailers, in countries such as Bangladesh and India.

To glean conditions for textile workers in British factories, Dispatches sent a reporter to work undercover in a number of factories in Leicester.

Channel 4 said there were multiple breaches of the ethical trading and the retailers’ own codes of conduct.

Conditions were “dangerous,” it said, describing one workshop thus: “The basement unit was cramped, over-heated and inadequately ventilated, with unsanitary toilets, dirty staircases and poorly lit corridors. With the greatest risk being fire, his only fire exit was completely blocked.” There were no clean facilities for providing drinking water.

There was no training or safety guidance before the reporter was set to work and sewing machines were not equipped with appropriate safety guards.

At both factories, the reporter was warned he would lose his job unless he worked faster. He was paid cash in hand of £2.50 by one employer and £3.33 an hour by another; the adult national minimum wage is £5.93. Textile workers said there were other similarly exploitative workshops nearby. Neither factory asked for documentation to check the worker’s legal status, citizenship or right to work in the UK.

A large number of the workers are Asians on student visas who are not supposed to be working, according to Gurjeet Samra, a Sikh elder who works with the Indian community in Leicester. Asked what life was life inside such factories, he replied: “It’s like slave labour, it’s like going back a hundred years in the way they treated. The people are so helpless they just got to do whatever they can... the bosses can shout at them or they can insult them, and these people can’t do anything about it.”

Channel 4 said: “With British consumers keen to buy the latest designer looks at cheap prices, this film exposes the real human cost behind high street fashion.”

Sir Philip’s Arcadia, which owns Bhs, said the group did not own or control any factories. But, it added, Channel 4's investigation had “found a supplier had clearly breached our strict code of conduct”. Arcadia said it used thousands of factories across many countries, which were regularly audited.

"The supplier concerned was contacted immediately and an investigation is underway,” it said, adding that any goods produced by the factory were being withdrawn from sale.

Arcadia added: "This incident is extremely disappointing as we are currently endeavouring to support UK manufacturers. In this case this has sadly backfired but it will not stop our efforts to continue to try to use UK manufacturers where possible."

C&A said it had launched an immediate investigation into the programme’s findings. However it added it did not have a contract with the factory investigated, saying the work had been sub-contracted.

Philip Chamberlain, its head of sustainable business, said: “The working conditions described are unacceptable to us. For this reason we have decided to suspend our business relation with the respective manufacturing supplier.

“New orders will not be placed with this contractual supply partner, until such time that a credible action plan has been presented to an accepted by C&A, in terms of his ability to fully control his supply chain in the future.”

Peacocks said: “We won’t be commenting before the programme because we don’t know what the full facts are.”

A spokesman for Jane Norman said that it had "come as a complete surprise and disappointment to find out that one of our suppliers engaged a factory that was apparently acting illegally and engaging in unfair and unlawful employment practices." The firm said that it was investigating the Dispatches allegations fully.

New Look thanked the investigators and said it shared “concerns about factory conditions in Leicester”. After carrying out its own investigation, the company met with its UK suppliers to remind them of “the risks linked to sourcing from UK factories, including poor health and safety, lack of transparency, lack of records, risk of illegal workers, minimum wage violations, unpaid and witheld wages, lack of proper contracts, sick pay and holiday pay”. New Look added that it did not allow sub-contracting in its supply chain.

Peacocks is also launching its own investigation. A spokesperson said: "Peacock's is committed to ethical business practice in all our operations... We are taking the issues raised very seriously and have immediately launched a full investigation."

Knitwear firm allegations

* Since relatively few garments are now produced in the UK, sweatshops scandals of late have primarily centred on Asia. But last year a BBC documentary alleged that a Manchester knitwear company – which supplied the retailer Primark – had employees who worked up to 12 hours a day and were paid £3.50 an hour. An undercover reporter said she was offered a job without being asked her name. An Afghan man said he was an asylum-seeker and had been working illegally in the country for three years. The film said the company had deliberately deceived Primark during an audit, which the firm denied. Primark said last summer procedures at the company had been improved to comply with immigration law, but it was still concerned about hours of work and pay.