Nazmul, 24, whose job is to stick pins into shirts, said he regularly worked more than 80 hours a week, with only one day off a fortnight. With overtime he makes 2,400 taka (£17) a month. "If a big order comes in we have to work. [In Britain] you get three-for-two offers. It is we people who have to make the third shirt for you. There's no choice. We just get shouted at. There are others who will take my place if I do not work."

Women, who make up two-thirds of the workforce, are particularly vulnerable. Another worker, Veena, 23, said she was accused of stealing a piece of cloth and sacked after complaining of sexual harassment. "I did not steal but I refused to do what the manager asked me [to do]. There is no union. Who can I complain to? Who will get my job back?"

War on Want says bargains in Britain, such as jeans for £3 and cocktail dresses for £6, are possible only because retailers wrench lower prices from suppliers in Bangladesh who get clothes stitched at the lowest possible cost.

The country has the cheapest garment workers in the world, with wages halving in real terms in the past 10 years. Experts say a living wage in Bangladesh would be 3,000 taka, well above shopfloor salaries in an industry of 2 million employees, despite massive street protests in September. Factories disgorge thousands of workers into huge slums constructed of bamboo, tin and concrete above fetid inky-black lakes.

Salma, 21, lives with two other girls in a tiny room in Begun Bari slum. Her basic wage is 1,150 taka a month for 48 hours a week as a shopfloor assistant making Primark clothes. By working to 3am she can double that. A factory job is one of the few socially acceptable ways for a woman to earn a living in a conservative Muslim country. "It is a hard life. I am shouted at. I prefer this to the village where [women] are not allowed to work."

There are dangers, however. After garment factory collapses and fires in Bangladesh left nearly 100 workers dead this year, safety has become an issue. War on Want claims emergency exits are often locked. Louise Richards, the charity's chief executive, said UK prices were at "rock bottom" only because of exploitation. "The companies are not even living up to their own commitments."

Factory owners said the $8bn a year clothing export industry was under intense scrutiny by foreign buyers but there was no extra cash for "social improvements".

Mohammed Lutfor Rahman, vice president of the Bangladesh Garment Manufacturers & Exporters Association, said western companies had imposed codes of conduct and sent inspectors to enforce the rules, counting fire exits and auditing overtime records. "I am asked about how many light bulbs we use in the factory and where is our toilet? But who pays for these things? The buyers' profits are going up. But if we ask for more money for improvements they say China is very cheap. It is a threat to move the work somewhere else."

Mr Rahman, whose factory sells mostly to German companies, took the Guardian on a tour of one of his units. On the third floor, rows of women stitched jackets under fans. "It is too cramped now. We are moving outside Dhaka."

Companies mentioned in the report said they had not seen it but took the issue seriously. Chris McCann, Asda's ethical standards manager, said he hoped the charity would share the findings so "we can do something about it". He said if he could identify the factories, there would be an audit of labour practices. "We have a clear policy and commitments. If these are violated we will investigate and expect the issues will be resolved. If people are being abused then frankly it is unacceptable."

A Tesco spokesperson said: "Tesco offers affordable clothing to UK customers - including many low-income families - but this is not achieved through poor working conditions in our suppliers' factories. All suppliers to Tesco must demonstrate that they meet our ethical standards on worker welfare, which are closely monitored. Our suppliers comply with local labour laws, and workers at all Bangladeshi suppliers to Tesco are paid above the national minimum wage."

Geoff Lancaster, head of public relations at Primark, said the company had been involved in trying to raise standards in Bangladesh and would investigate. It denied it was cutting costs so British shoppers benefited from cheap prices. "We use huge volumes, deal directly with suppliers cutting out the middlemen and do not advertise. That's how we get best value."

Names of workers have been changed.