When 15-year-old Kristy Bamu left his parents in Paris on 16 December 2010, he was looking forward to spending the Christmas holidays with his siblings, visiting their sister and her boyfriend in London.

On Christmas Day he was found by paramedics in the bathroom of an east London high-rise flat. His body had been mutilated, teeth were missing and he was covered in deep cuts and bruising. In the last four days of his life he had suffered acts of unspeakable savagery, doled out by a man he called "uncle" and one of his own sisters.

Why? Because Eric Bikubi, a powerfully built football coach, and Magalie Bamu were convinced the boy was a witch, possessed by spirits who wanted to bring evil into their home. On Thursday they were convicted of murder. They had earlier admitted actual bodily harm against Kristy's sister Kelly and a younger sister, who cannot be named.

The judge, Mr Justice Paget, exempted the jury from jury service for the rest of their lives because of the "strain" of the trial, adding it was a case "every one of us will remember". The story of Kristy's last days is difficult to hear, and harder to comprehend. He was starved and deprived of water and sleep, punched and kicked repeatedly, floor tiles were smashed over his head, his teeth were hit out with a hammer and a pair of pliers were used to twist his ear. Throughout the ordeal, his siblings – two of whom were themselves accused of being witches by the couple and abused – were forced to watch or take part in the torture.

But despite the extraordinary horror of this case, African groups have warned that belief in witchcraft is increasingly common in some communities and that other children in the UK are "suffering in silence" after being branded as witches.

"We were concerned about this before this trial of Kristy Bamu," said Debbie Ariyo, executive director of Africans Unite Against Child Abuse (Afruca), who added that a boom in pentecostal churches was leading to more children being accused of witchcraft. "This is not a problem with all pastors or all churches, but the branding of children as witches is not abating. It is a growing problem. There are so many children suffering in silence."

In response to the case the Victoria Climbie Foundation announced on Thursday that in April it would be launching a cross-border initiative with Kelly Bamu and some of the teenager's friends who would work in schools and in communities to highlight the dangers of ritualised child abuse and witch branding. "A lot of awareness raising has been done, but this is also a problem in other EU countries such as Belgium," said director Mor Dioum.

The 83 incidents uncovered in the past decade only scratch the surface of a hidden crime, according to Detective Superintendent Terry Sharpe, head of the child abuse investigation command at Scotland Yard.

An average of eight children a year in Greater London are victims of abuse based on witchcraft-style exorcisms, but this only reflects cases resulting in police investigations.

Sharpe detailed the horrific abuse, including being beaten or forced to drink unknown liquids, starved or deprived of sleep, blindfolded and having their hair shaved off.

Kristy Bamu was subjected to many methods of torture. It began with a simple accident when Kristy, waking in an unfamiliar bed, wet himself. Bikubi, finding the underwear, accused the boy of being possessed by kindoki – the word for witchcraft in the Congolese Lingala language. This is a recognisable trigger in the Democratic Republic of Congo (DRC), where both Bikubi and Bamu were born, according to Dr Richard Hoskins, an expert in African religions.

"The trigger that needs to occur for people to think someone is possessed by kindoki can be anything out of the ordinary. Bed-wetting is a classic example of this," he told the Old Bailey trial.

For the next four days, Kristy and two of his sisters – Kelly, 21, and an 11-year-old – were accused repeatedly of being witches. They were forced to fast and stay up all night chanting prayers. In horrific evidence, which reduced members of the jury to tears, the court heard that Bikubi soon fixated on Kristy. Over several days, he beat him around the body with a metal bar used for weights, shoving the end of it into his mouth and dislodging a tooth. The football coach headbutted and hit the teenager, smashing bottles and then heavy floor tiles – bought to redecorate the flat – over his head. When paramedics arrived at the flat on Christmas Day and found the child drowned by the side of a bath, they found his blood all over the flat and an "armoury" of weapons that had been used to torture him.

Witchcraft had preoccupied Bikubi from an early age. Speaking to a forensic psychiatrist, Dr Tim Rogers, in Pentonville prison, he explained as a child in the DRC he saw rats and other "abnormal visions" and was isolated as a result. Bikubi – who moved to London when he was seven – believed he was "the chosen one", with a special ability to sense other people's spirits.

Thomas Bikebi, executive director of the Congolese Family Centre, said that for some believers once the presence of evil spirits had been "confirmed" – often, but not always by a pastor – the "punishment" was seen to be imposed on the spirit and not on the child. "When you force the child to fast, people believe you are starving the spirit, not the child. When you beat, you are beating the spirit," he said.

The widespread belief in kindoki could even lead those who believed in the evil power of spirits to support the actions of Bikubi and Bamu, he said. "There are people within the community who will say that this pair did the right thing, they killed a witch," Bikebi said.

Common problems experienced by children – such as autism, epilepsy, dyslexia or even simple naughtiness – could trigger accusations, said Ariyo, with children living away from home or in domestic servitude most likely to be targeted.

"It is heartbreaking. Some of these children have a disability but pastors are saying they are possessed," she said. "This can prevent parents going to see professionals and getting the help they need."

There has been a rapid growth in African churches in the UK. According to UK Church Statistics, 670 pentecostal churches opened between 2005 and 2010, taking the total to 3,900 – that figure is expected to rise to 4,600 by 2015. As a result, Ariyo fears that "witch-branding" is on the increase. Afruca dealt with a dozen cases of ritualised abuse last year, and said the national figure was likely to be far higher.

A social worker with more than 30 years' experience with African communities in London said that many cases were "going under the radar" and blamed "rogue" pastors for the spread of branding and ritualised abuse. "It is spreading like bushfire because it is a source of income," she said. "If you can charge £500 for an oil that is going to 'cure' a child of evil spirits, you are going to make money. Pastors can be very powerful people, and we have to educate the bad ones that there are other ways of making money than playing on people's ignorance."

Police believe Bikubi may have visited Nigerian preachers in north London but his local churches denied knowing him. However, access to the internet and satellite channels meant the influence of pastors based in countries such as Nigeria, Angola and the DRC was increasingly pervasive, Bikebi said.

"You have pastors online who tell people that they just need to send money and touch the table in front of them and the evil spirit will be banished and all their problems will be fixed."

One video on YouTube, featuring the well-known Nigerian preacher David Oyedepo, reveals the power of some church leaders. A teenage girl kneels before the pastor, and after saying she is a "witch for Jesus" he calls her a "foul devil", slapping her violently across the face while the congregation cheers. In a later video, he can be seen boasting: "I slapped a witch here last year."

Many African churches work with agencies to educate preachers about UK laws concerning child protection, according to the Churches' Child Protection Advisory Service. The organisation has trained 4,000 preachers from African churches since 2007, but argues that local boards for safeguarding children must do more to know what churches are operating in their areas.

A lack of any requirement to note religion in child abuse cases was likely to lead to under-reporting, said its chief executive, Simon Bass.

"Like female genital mutilation, there is likely to be under-reporting in this area, so we have to work with statutory agencies and across the church spectrum to ensure more children do not suffer this type of abuse," he said.

Project Violet, the Metropolitan police's faith-related child abuse unit, has a team of three dedicated officers working with communities, schools, religious leaders, and the medical profession to raise awareness, said Sharpe. "We have ongoing community engagement … and are in the process of increasing awareness of Project Violet within the Met police so that frontline officers who attend incidents are aware of these particular issues," he said.

Critics argue the unit has been seriously neglected in recent years, after making an initial impact when it was established in 2005 as a response to the abuse of child B – an eight-year-old Angolan girl who was beaten, cut and had chilli rubbed in her eyes for being a "witch".

"Project Violet is a shadow of its former self," said Ariyo. "The police are not engaging enough with faith groups, and there is a gap that needs to be filled. We shouldn't shed tears after another child has been killed. We need to act now."

Afruca is calling on the government to make branding a child as a witch illegal, and is calling for greater monitoring of churches and preachers. "If we don't push home the idea that calling a child a witch will have grave consequences, then we will continue to have these kind of cases," said Ariyo. "We don't seem to be learning from these cases. People cry crocodile tears and then nothing happens."

On Christmas Day 2010, Kristy's killer spoke to the boy's father, Pierre, accusing the 15-year-old of being a witch and threatening to kill him. Kristy told his father: "Dad, come and get me, or otherwise Eric will kill me". The court heard Pierre Bamu, dismissed the boy's fears because he could not imagine Bikubi causing any harm to the children, who had stayed with Bikubi before. A few hours later, Kristy was dead.