A couple have been jailed for life for torturing and drowning a teenage boy they accused of being a witch.

Kristy Bamu, 15, was killed by his sister Magalie Bamu, 29, and her partner, Eric Bikubi, 28.

He died in a bath at their tower block flat in Newham, east London, on Christmas Day 2010 after days of being abused.

He had come to London from Paris with his two brothers and two sisters to spend the festive season with Magalie Bamu.

But things turned sour when the couple, who were said to be obsessed with kindoki, the word for witchcraft in their native Democratic Republic of the Congo, accused him of putting spells on a younger child.

Football coach Bikubi and Bamu were found guilty of murder at the Old Bailey last week.

Bikubi was ordered to serve at least 30 years and Bamu a minimum of 25 years.

Judge David Paget told the couple the case was particularly serious and involved sadistic behaviour.

"It was prolonged torture involving mental and physical suffering being inflicted before death," he said.

He told Bikubi that he accepted that brain damage may have made him more inclined to believe Kristy was a witch and a threat to the younger child.

But Judge Paget added: "The belief in witchcraft, however genuine, cannot excuse an assault to another person, let alone the killing of another human being."

He told Bamu he did not accept her denial of belief in witchcraft, nor that she was forced to attack Kristy by Bikubi.

"It is only explicable if you shared Eric Bikubi's belief. It provides some explanation for what happened, but it does not excuse it," he told her.

The judge said the couple had also attacked Kristy's sisters but he would not pass separate sentences for that.

He added: "The ordeal they were subjected to almost passes belief."

Henry Grunwald QC, for Bikubi, said: "What happened would not have ended as it did had it not been for Mr Bikubi's mental impairment."

Philippa McAlasney QC, for Magalie, said: "Not only has she has lost her entire family, she faces a solitary life in prison."

Kristy's father, Pierre, said in a statement: "Kristy died in unimaginable circumstances at the hands of people he loved and trusted – people we all loved and trusted.

"I feel betrayed. To know that Kristy's own sister, Magalie, did nothing to save him makes the pain that much worse."

Detectives said other children in Britain had been subjected to terrible ordeals after being accused of witchcraft, and children's charities and campaigners called for more to be done to make carers and churches aware of possible abuse.

The court heard that Kristy was in such pain after three days of fasting and being attacked that he "begged to die" before slipping under the water.

He had been struck with knives, sticks and metal bars, as well as a hammer and chisel.

After he refused to admit to sorcery and witchcraft, his punishments in a "deliverance" ceremony became more horrendous.

Bikubi forced the youngsters to pray for deliverance for three days and nights and deprived them of food and water.

On Christmas Day he placed them in the bath for ritual cleansing.

But Kristy was too weak. He begged to die and slipped under the water.

His sisters, aged 20 and 11, were also beaten but escaped further attacks after "confessing" to being witches.

Kristy was singled out after wetting his pants. He was struck in the mouth with a heavy bar and hammer, knocking out his teeth.

Ceramic floor tiles and bottles were smashed on his head and a pair of pliers used to twist his ear.

The terrified siblings, who also included a 13-year-old boy and an autistic brother aged 22, were made to join in the torture.

At one point, Bikubi told the youngsters to jump out of the window to see if they could fly, the court heard.

They looked to their older sister to save them, but instead Bamu encouraged Bikubi and beat Kristy until he also confessed to witchcraft.

Sister Kelly, now 21, broke down several times in court as she relived the terror.

She said: "It was as if they were obsessed by witchcraft. They decided we had come there to kill them."

Kelly added: "Kristy asked for forgiveness. He asked again and again. Magalie did absolutely nothing. She didn't give a damn. She said we deserved it."

Kristy had 130 separate injuries and died from a combination of being beaten and drowning.

Brian Altman QC, prosecuting, said: "In a staggering act of depravity and cruelty, they both forced the others to take part in the assaults upon Kristy.

"As Kristy's injuries became ever more severe, he even pleaded to be allowed to die.

"Kristy was killed in the name of witchcraft. It is hard to believe in this day and age anyone can believe someone was practising witchcraft."

Bikubi pleaded guilty to two counts of causing actual bodily harm to the girls. Bamu denied the assaults but was found guilty.

Scotland Yard has investigated 83 cases involving abuse resulting from ritualistic or faith-based beliefs, and brought 17 prosecutions, over the last 10 years.

Detective Superintendent Terry Sharpe said: "This is a hidden and under-reported crime and therefore difficult to deal with in terms of protecting potential victims from harm."

Save the Children's head of child protection, Bill Bell, said: "This case must serve as a wake-up call to governments and local authorities to do more to prevent this kind of terrible abuse from happening to children in future."