The body was found without a head

The headless and limbless body of the victim, called Adam by Scotland Yard officers, was found in the river near Tower Bridge in London in September 2001.

New evidence found in the boy's lower intestine has been identified as the highly poisonous calabar bean which police believe may have been used to subdue him.

A file is now being prepared for the Crown Prosecution Service (CPS) raising the possibility of extraditing suspects from more than one country, according to BBC crime correspondent Neil Bennett.

Police believe Adam was aged between four and six and was alive when he was brought to London from Nigeria.

We are now closer to charging people with this crime Scotland Yard spokesman

He is thought to have become the victim of a ritual killing days later.

In a statement on Thursday, the Attorney General Lord Goldsmith said he would like to lead the prosecution team if the case does go to court, which would be the first time he has done so.

While it would not be part of his remit, the statement said Lord Goldsmith wanted to send "a clear message to those who deceive, coerce or force vulnerable people to leave their homes for a life of exploitation and misery - or worse - that they will not escape justice".

Detective Inspector Will O'Reilly, who is leading the Metropolitan Police inquiry, said he had never given up hope of solving the crime.

He told the BBC: "You are always optimistic. I knew it was a challenge.

"According to the FBI and people we spoke to in the early days this was almost unsolvable.

"You did not know who the victim was, when it happened or how it happened."

He added there may have been other ritual killings which were not discovered or recognised as such before the investigation raised awareness of the practice.

"With two turns of the tide Adam's body could have been out in the North Sea and we may never have found him," Mr O'Reilly said.

"We could have had them (ritual murders) before, we just might not have identified them as such."

Officers travelled to Nigeria to try to trace the boy's family after forensic examination of his bones found he had been brought up in Benin City.

A botanist at Kew Gardens identified the calabar bean that was found inside Adam's intestine.

A spokesman for Scotland Yard said: "We are now closer to charging people with this crime."

It's My Story: Finding Adam is on Radio 4 at 2000 BST on Thursday