The short, stooped man known as Kredens worked from dawn till dusk – laying bricks, plumbing, plastering; even sewing furs. His masters beat him often, and when they no longer needed his labour they sold him on to new slave masters – shifting him from worksite to worksite, city to city. Sometimes they threw him scraps of food or a crumpled banknote. Sometimes they paid him nothing at all.

Kredens had struggled through a 30-year ordeal as a modern slave not in some lawless failed state, but in cities across Britain. Tempted away from his Polish hometown on the promise of a good job with decent pay in the UK, he had fallen into the hands of a brutal human trafficking ring. Even when he managed to run away, with nowhere to go he was soon recaptured by slave gangs who prey on rough sleepers on Britain’s streets. For decades, it seemed he was locked in a cycle of homelessness and enslavement from which there was no escape.

But then, as he neared his 60th birthday, it seemed there might be a way out. “You are not alone,” the then home secretary Theresa May promised victims as she unveiled the Modern Slavery Act 2015 – a landmark law designed to stamp out human trafficking in Britain. “We are here to help you.”

The pledge was simple: If Kredens plucked up the courage to come forward and tell the police about his ordeal – and if his story was believed – he would be given official recognition as a victim of modern slavery, entitling him to government protection under the new law. So Kredens came forward. Then, in the temporary shelter of a safe house, he waited to learn his fate.

The good news came on 11 October last year: An official letter congratulated Kredens on having gained victim status under the Modern Slavery Act. That entitled him to another couple of weeks in the safe house while he got himself back on his feet. But, just as he was beginning to contemplate his new life as a free man, the bad news hit.

The Home Office had denied Kredens the right to remain in Britain. As a former slave, he could not show that he had been gainfully employed during his time in the country – rendering him an “illegal immigrant” in the government’s eyes and shutting him off from any access to state welfare, including housing. Without a roof over his head, he would once again be at the mercy of the trafficking gangs targeting rough sleepers. Because of the very nature of the exploitation the government had just officially recognised, he was being denied help and cast back into danger.

A BuzzFeed News investigation today exposes how gaping holes in the government’s flagship modern slavery strategy are leaving victims like Kredens homeless, destitute, and vulnerable to re-trafficking or deportation – even after they have been officially granted government protection. The investigation reveals how trafficking gangs – emboldened by the government’s failings – are preying in growing numbers on Britain’s swelling homeless population in plain sight of the authorities. As well as migrants smuggled in from abroad, the victims include British citizens, snatched from the streets in broad daylight.

To expose the scale of the problem, BuzzFeed News has surveyed 75 homelessness and trafficking charities; interviewed dozens of police officers, government advisers, lawyers, charity workers, and rough sleepers; conducted weeks of surveillance to gather secret footage of “suspected traffickers”; visited “tent cities” where exploited workers are sleeping overnight; and interviewed six victims – including the first media interview with a victim of the notorious Rooney family gang, who held 18 homeless and disabled slaves captive for over a decade, and two victims of a prolific trafficking gangmaster who enslaved scores of workers in the UK.

It can today be revealed:

● Exclusive data shows hundreds of homeless people have been captured by slave gangs in the UK in the past three years – but that is likely a fraction of the total because many cases are never documented, with the government, local authorities, and many police forces failing to keep any data on the trafficking of rough sleepers.

● In one case currently under investigation, BuzzFeed News has learned a trafficking network is suspected of controlling up to 250 victims in Britain, many of whom were homeless.

● Britain’s slaves have been kept in fetid, rat-infested caravans, sometimes without heating, bedding, or running water; chained up; beaten; and even locked in outdoor “rabbit hutches”. Their captors steal their ID documents, often using them to commit fraud and other crimes, and extract trumped-up “debts” from their illegally low wages.

● Traffickers are approaching homeless people at soup kitchens, shelters, and rough-sleeping sites across the UK and luring them into slavery with the promise of cash, drugs, or alcohol.

● BuzzFeed News identified seven suspected trafficking hotspots in London, Birmingham, and Bradford and obtained secret surveillance footage of vans picking up destitute and homeless men from street corners and taking them to local worksites.

● Victims who approach the Home Office for help are often rounded up and deported as part of the government’s policy of making Britain a “hostile environment” for “illegal immigrants” – making many afraid to come forward.