England, United Kingdom - In a safe house in North West England, 19-year-old Lukas* is reliving his own personal hell over and over again.

"Over the two years, I was given accommodation, but often I wouldn't be paid at all and couldn't afford to eat," he recalls, unable to keep eye-contact or seemingly comprehend his experience.

Originally from Lithuania, he is one of the thousands of victims of the modern-day slave trade who suffer from severe mental illness, including experiencing intense paranoia that his one-time tormentors will come and take him back.

Forced to work 12 hours a day across the United Kingdom delivering packages, Lukas was paid as little as 20 British pounds, or $24, a week - if he was paid at all. He recalls having to live in a van for a number of weeks.

"If other people asked where their money was, they'd be beaten," Lukas says.

The British government estimates that there are around 13,000 people in modern-day slavery in the UK in 2014. And it's on the rise: Police and other authorities identified 3,266 people last year thought to have been the victims of modern-day slavery, up from 946 in 2011, 46 percent of whom were identified as male, and many had mental health issues.

At the safe house, victims are given a 45-day minimum reflection and recovery period, which is often extended to over 90 days, to help them consider their next step. During this time, they are offered financial, legal, medical support and counselling, as well as help finding a job and accommodation.

While many people manage to move on successfully and turn their lives around during the period, mental illness in others puts them at a severe disadvantage, often leading them to destitution and a risk of re-trafficking.

INTERACTIVE: Britain's modern slave trade

Dealing with PTSD

In the safe house, Lukas, who remains positive, is applying for a university course, while a Greek victim, Giorgos, who was homeless before being forced to work producing cannabis, has saved up a deposit, got a job and is ready to move on into society since living in the house.

Yet, in the same house, Matthew, a British victim, lies traumatised. He was kept in a caravan and fed cocaine and alcohol to control him by his traffickers, forced to engage in sex parties and homosexual activities despite being heterosexual.

He now frequently misses counselling appointments after self-medicating on drugs and alcohol.

Sam Baxendale, who runs the house for the Medaille Trust, an organisation helping victims recover from human trafficking, told Al Jazeera of another victim, who was sectioned - admitted to a hospital - after repeatedly running into the street naked and lying there, often approaching women and children and trying to talk to them.

"He's a lovely man," Baxendale explains, "but he had to be sectioned; he was a threat to himself and others."

In other cases, safe house workers have had to leave medical kits in victims' bedrooms, so that if they self-harm, they can bandage themselves up. At the lower end of the spectrum, some victims simply can't get out of bed, experiencing a deep malaise.

They often stay because they're threatened, their families are threatened at home, because they're told they can't trust the authorities and police... Major Anne Read, The Salvation Army

Symptoms vary from case to case, but typically there is a "sense of hopelessness, anxiety, fear, fear of the future and what will happen, suicidal tendencies and self-harming", says Phillipa Roberts, legal director at Hope for Justice, a charity which helps victims from rescue to recovery.

A report published this year by King's College London backs her up, finding some 40 percent of male victims reported high levels of depression, anxiety or post-traumatic stress disorder ( PTSD ).

"Our clients often present with complex forms of PTSD," says Rachel Witkin, head of counter-trafficking at the Helen Bamber Foundation, which supports people who have experienced human rights violations and extreme interpersonal violence.

"They may experience flashbacks, which means that they are 'seeing' and 're-experiencing' traumatic things happening all over again," says Witkin. "I find that clients can become exhausted and withdrawn from having recurrent panic attacks, nightmares and intrusive thoughts and memories.

"Their PTSD is complex because it is a reaction to prolonged and repeated interpersonal violence which is inflicted over a period of time."

For Witkin, this is exemplified in victims of trafficking who have suffered sexual abuse.

"In my experience, physical ill-treatment, including sexual abuse and rape, is often a central component at various stages or throughout the trafficking experience, for men, as well as for women. Trafficking is a form of obtaining complete control over a person, and the use of rape as a weapon is a powerful method of control," Witkin says.

"Men who have been trafficked primarily for other reasons - no matter what the trade or industry - have often also suffered sexual abuse and rape."

Psychological captivity, conditioning and subjugation of victims can significantly affect victims' future mental health, Witkin adds.

Major Anne Read, director of modern slavery at The Salvation Army, which has the government contract to support victims, explains that modern slaves are not often chained up any more, but still suffer from deep psychological abuse while living among the rest of the population.

"They often stay because they're threatened, their families are threatened at home, because they're told they can't trust the authorities and police, [or] trust the immigration system that may deport them, and if they try and escape, not only might they be victim of some kind of physical abuse, but their families could be as well, and if they went to the police, the police would also abuse them," Read says.

Furthermore, Roberts explains that "there is often psychological control through threats and verbal abuse, or additionally we see commonly grooming - 'you'll get paid eventually', so people think they'll get paid at some point or conditions will improve."

INTERACTIVE: Modern-day slavery - The numbers

Cycle of exploitation

Nicolae, a 29-year-old Romanian, continued working under the difficult conditions because he hoped he'd eventually get what he was promised. He recalls how man pulled up on the street near his home city one day, and offered him a job in the UK for 700 pounds, or around $860, a month. His family was living in extreme poverty and some days unable to feed themselves, so he jumped at the opportunity.

"I ended up being paid just 300 pounds [$370] per month for almost a year, working 12-hour night shifts daily in a factory. I needed that money, so I stayed," he explains to Al Jazeera with tears in his eyes.

"Initially, I was told I would be given food, but for three months, I had to buy my own food. I thought I shouldn't spend too much on food, so I drank a lot of coffee. Before coming here, I didn't look like this. I'm so thin now."

Accommodated under the watchful eye of his captor, his trafficker confiscated his documents in an effort to keep him there, but also to exploit him further.

SLAVERY: A 21st-Century Evil

"He took my ID card. He used these documents to apply for bank accounts and many other things. We would go out together. He said, 'Come with me, I will help you get some documents,' but he'd take me to banks or shops to take mobile phone contracts, all sorts of cunning techniques to exploit me," Nicolae recalls.

"In the end, he beat me up, really bruising my face. I couldn't stay there any more, so I slept on the streets because I didn't know where to go."

Now, without his documents, Nicolae cannot work and has to return to Romania to reapply for them before attempting to come back to the UK. The staff at the safe house say he is deeply unhappy and losing all hope, drinking to excess to cope with his trauma.

"You see an incredible amount of alcohol use as a coping mechanism or after effect," Mike Emberson, chief executive of the Medaille Trust, says. "People want to block out awful experiences, deaden the pain and the memories with alcohol."

"Some feel they have nothing else to do in their life, so [they] turn to self-medication, lying in bed and drinking themselves to sleep. It is extremely debilitating," Baxendale says. "Some people are generally beyond the help on offer, and will go back homeless, many of whom are thought to fall straight back into the hands of the people who tricked them."

This often happens because, after the reflection and recovery period, many men end up homeless or in poor accommodation very quickly as they have no special right to accommodation above and beyond what other homeless people are entitled to.

Beyond help

People who are already suffering from mental health problems are even more likely to find themselves in these situations. They make more attractive targets for traffickers who look out for the vulnerableon the streets, shelters and other places they are likely to be, leading to a devastating cycle.

Even if victims find housing, "one of the biggest challenges for victims afterwards, is living in poverty or suffering destitution," Witkin explains.

"Poor housing and accommodation shared with strangers can put victims of trafficking, who are already traumatised by their experiences, at risk of further harm due to their vulnerability and the fragility of their mental health," says Witkins.

While some charities in the sector believe steps are being taken to solve problems leading to destitution and help victims with mental health, for now, many remain at risk, with traffickers ready to pounce once again.

Benedek, 57, is one that could be at particular risk. An older Hungarian man, he was told he would be interpreting when he came to the UK, as he speaks English. However, he ended up at a carwash,and was provided with alcohol to keep him complicit.

He now grapples with mental health issues, violence and aggression, symptomatic of PTSD, often after drinking heavily.

Struggling with memory, Benedek appears to wonder why he is there: "I just want to get a job. I just want to work and get on, buy a house, live and work."

If you are someone you know in the UK is a victim of human trafficking, call the Salvation Army's 24-hour referral line at 0300 3038151.

*Names of victims have been changed for privacy and security purposes