After five years in the brothel, Labonni stopped dreaming of being rescued. Ever since she had been sold to a madam at 13 years old, customers had promised to help her escape. None had followed through. Over time, their faces began to blur together, so she couldn’t remember exactly who had visited before, or how many men had come by that day. There’s usually one every hour, starting from 9am.

“Sometimes I wake up and I don’t understand why I’m not dead yet,” she says.

Now 19, Labonni says she’s resigned to life – and death – in Mymensingh, a brothel village in the centre of Bangladesh. Here, between 700 and 1,000 women and girls are working in the sex trade – many of them against their will.

Girls as young as 12 sleep five to a room; their beds only cordoned off by torn cotton curtains. Music blares from heavyset sound systems and homemade liquor is poured from plastic bottles to numb the pain. Men swagger shirtless down the alleys looking for girls. Ten minutes of sex will cost them TK400 (about £3.66) – but it’s money that mainly lands in the pockets of those running the brothel.

Like the majority of girls in Mymensingh, Labonni was trafficked into sex work. On the run at 13 years old, she left her six-month-old daughter behind to flee the abusive husband she had been made to marry the year before, in a ceremony that took place on the same day she started her period. “I didn’t know where I was going,” she remembers. “I thought maybe I could find work in a garment factory.”

A woman saw her looking tearful in Dhaka railway station, and offered her food and a place to sleep for the night. Two days later, Labonni was sold by her to the brothel for about £180 and forbidden to leave.

Facebook Twitter Pinterest A graveyard for sex workers in Tangail. Photograph: Allison Joyce

Overnight, she became a chukri, or bonded sex worker – imprisoned within the brothel until she repaid hundreds of pounds in fabricated debts. “The madam who bought me said that I had to pay her back,” Labonni says in a flat voice. “She’d bribed the police to say I was 18 [the legal age for a registered sex worker] and told me I owed her more than £914. Then she confiscated my phone and locked me in my bedroom. She said that she’d hurt me if I tried to run away.” After two or three months, Labonni gave up trying to escape. “They always find you,” she adds.

A quick breakdown of the figures involved shows how girls like Labonni are a vital part of a hugely profitable business model for brothel owners in Bangladesh. For the past six years, since being trapped in the brothel, she has worked continually to pay off her phantom debt. Yet over those six years she has earned upwards of £46,500 for madams who enjoy lives of considerable luxury.

Until last year everything Labonni earned went to her madam. All she was given back was a £37 as a monthly allowance for food, clothes and toiletries. Labonni has now paid her original £914 “debt” back 50 times over.

Last year she was finally told she had paid off her debt, but she has yet to move on. Her mental strength is worn down by years of abuse. “I feel worthless,” she says. “My daughter doesn’t even know I’m her mum.” Even with her “debt” gone, she’s still obliged to pay half of her weekly earnings – approximately £78 – to the madams in exchange for electricity and a place to stay.

One of her regular customers, Mohammed Muktal Ali, is 30 years old. A married bus driver from the nearby town, he has been visiting Labonni every day for four and a half years, since she was 14. “All the girls here are helpless,” he says. “You can’t sell a boy to a brothel, but you can sell a girl because she has monetary value.” He doesn’t feel guilty for paying for sex with a trafficked teenager. “I am in love with Labonni. I’m 70% sure that one day I will rescue her.” Labonni doesn’t look up. “I don’t believe anything the men say to me any more,” she says later. “They all lie.”

Four floors down from Labonni’s bedroom, Farada, 33, says the number of trafficked girls has increased since she arrived at the brothel in 1999. She knows, she says, because she buys them. After 12 years entrapped in sexual slavery herself, she was given a girl as a gift by a customer eight years ago, moving from exploited to exploiter overnight. When the girl escaped, she bought a second, called Moni, for £137. “I paid £27 on cigarettes for the police, and they sorted all the paperwork,” she says, referring to the government-mandated certificates that state every sex worker is at least 18 and consents to engaging in prostitution. “Now the police charge more. It’s at least £450, which is very expensive, so the girls have to pay me back.” The younger the girl, the higher the bribe required by law enforcement, she adds.

These days, she makes about £187 every week from two girls, but says a third of that goes to local gang members who control the brothel.

Facebook Twitter Pinterest A child waits outside their mother’s room while she works. Photograph: Allison Joyce

The money being made in this single brothel is an indicator of the vast profits generated by the global trade in women and girls. Sex trafficking is an enormously lucrative business.

Academic Siddharth Kara advises the United Nations and the US government on slavery and has shown through his own research that sex trafficking is disproportionately lucrative compared with other forms of slavery. He estimates that sex trafficking creates half of the total profits generated globally by modern slavery, despite only accounting for 5% of all trafficking victims worldwide.

He told the Observer: “The return on investment for sex trafficking is around 1,000% compared with much lower returns in exploitation for construction, agriculture or mining. The immense profitability of sex trafficking is … driven by the minimal expense associated with acquiring victims and the fact that the victim can be sold up to 20 times a day, generating tens of thousands, if not hundreds of thousands, of dollars in profit per victim.”

Prostitution was legalised in Bangladesh in 2000, after the year-long detention of 100 sex workers by police sparked protests calling for the women’s freedom and equal rights. The women’s release heralded a new legal framework, but few protections.

Instead, the business of sexual exploitation has thrived in a country where women are oppressed in many ways. Across the country, one in five girls is married before her 15th birthday and only a quarter finish secondary education. Choice is a luxury few women here can afford.

While prostitution is legal, trafficking and forced labour are not. But poor enforcement of legislation in a country where women are easy prey means traffickers act with impunity. The Bangladesh government estimates that 100,000 women and girls are working in the country’s sex industry and one study reports that less than 10% of those had entered prostitution voluntarily. This investigation found hundreds of girls who spoke of being sold by strangers, family members or husbands without their consent.

In April the Dhaka Tribune reported that the conviction rate for people arrested in connection with trafficking is less than half a percent. While more than 6,000 people have been arrested in connection with human trafficking since 2013, only 25 were convicted. Last year only eight traffickers were convicted in Bangladesh.

Facebook Twitter Pinterest A girl of 14 waits for customers in a Jessore brothel. Photograph: Allison Joyce

While many girls sell sex from their homes or the street, more than 5,000 women and girls are split between 11 huge brothels countrywide. Some dating back hundreds of years, each brothel is registered with the government and monitored by the local police. Here, a triumvirate of powerful institutions – government, police and religion – watch over and approve the rape, enslavement and abuse of hundreds of thousands of prepubescent girls.

“The Bangladeshi police know everything that takes place in the brothels,” says Azharul Islam, programme manager of Rights Jessore, a local non-governmental organisation working to rehabilitate trafficked children working in the sex trade and return them to their families. “The brothel owners are involved in gangs, and our political leaders and law enforcement are involved in those gangs, too.” Corrupt government officials profit by accepting bribes and sexual favours in exchange for turning a blind eye to the abuse.

As part of this investigation, more than 20 underage girls in four of the brothels showed us their notarised certificates stating they were over 18. One girl admitted she was still 13. “It’s law enforcement, it’s the local mafia,” says Mahmudul Kabir, Bangladesh country director for the Netherlands-based NGO Terre des Hommes. “And it goes through the entire chain of power.”

The steady stream of women and children being trafficked into Bangladesh’s sex industry means that the girls are disposable to those making money out of them. The numbers killing themselves has reached a point where at least two brothels in central Bangladesh – Kandapara, on the on the outskirts of Tangail, and Daulatdia, on the banks of the Padma river – have had to built private graveyards to cope with the dead.

“There’s about one death a month,” says Shilpi, 57, who was sold to Daulatdia brothel in 1977. “It never used to be this many.” These days she conducts the funerals: washing each body before leading a team of 12 brothel guards through the thicket of weeds that shrouds the burial grounds; finally reciting a short prayer over the grave. She doesn’t know how many girls are buried there. She lost count after 100. “For a while, we tied a stone around their necks and threw the bodies in the pond,” Shilpi adds. “But sometimes they floated to the surface, so we had to find land.”

Facebook Twitter Pinterest A 15-year-old girl gets married. Many victims of trafficking said they were sold to brothels by their husbands, without their consent. Photograph: Allison Joyce

In Mymensingh, there’s no such graveyard – but not from lack of need. Instead, bodies are carried out to the countryside at nightfall; buried in unmarked graves by torchlight.

Public graveyards aren’t an option: the stigma that surrounds sex workers in Bangladesh forbids their burial in municipal ground. “Here we are shameful, bad women,” says Shilpi. “If a girl kills herself, people say it’s good riddance – it’s just a quicker way for them to get to hell.”.

Labonni has also tried to kill herself several times. “I’ll probably try again one day,” she says, sitting on the floor of the concrete cell that passes as a bedroom: her customers’ phone numbers are scratched into the wall. Meanwhile, she cuts herself daily.

Such deep-rooted mental health problems are endemic among Bangladesh’s bonded brothel workers, and make it harder for them to move on even when their “debts” are paid. Though there is little mental health support for the women, there is evidence that when it’s provided, it helps. One organisation working to rescue and rehabilitate underage trafficking victims is the Bangladesh National Women’s Lawyers’ Association. “When they first arrive at the home, they’re scared,” says BNWLA psychologist Sadia Sharmin Urmi. It takes consistent counselling to help them move forward, but within three months, she sees progress. “They know they are safe. That means a lot.”

For Labonni, the idea of ever getting help feels unlikely. “All my life, people tell me to have sex so that they can make money from it. How much do I have to earn to be free of this life?”

Escape now takes the form of daily video calls with her daughter, who lives with her elder sister in Dhaka. “I can’t raise her here and that hurts me, but I know she’s happy,” Labonni says. “One day, when she’s old enough, I would like her to know I’m her real mum.”

Fuller Project for International Reporting and Girls Not Brides

supported this investigation. Additional reporting by Ali Ahsan and Pragna Chakma

40.3million

The number of people living in some form of modern slavery across the world. More than half of the victims are in forced labour. Sex trafficking is a form of modern slavery that involves the use of coercion, abduction, fraud or force to profit from someone’s sexual exploitation.

99%

of the estimated 4.8 million sex trafficking victims across the world working in the sex industry are women and girls.

13,000

people are estimated by the UK government to be the victims of modern slavery in Britain today.

70%

of the world’s 4.8 million sex trafficking victims are in the Asia and Pacific region.

$150bn

The amount generated by the modern slave trade every year, according to UN estimates. Sex traffickers can earn up to £29,000 per victim.

• This article was amended on 7 July 2019 to correct the headline, which originally referred to brothels in Dhaka.