The body was found by a member of the public in the river near Tower Bridge on September 21 at around 4pm.
Although police have never been able to identify him, he is referred to as “Adam”, and detectives believe he would have only been five or six years old and from Nigeria.
He was believed to have been trafficked into the UK, possibly from Germany, and his head and limbs had been cut from his torso. He was wearing just a pair of orange shorts.
It is believed his death may have been a ritualistic killing, with the cause of death found to be violent trauma to his neck area.
Officers have continued to review the case over the past two decades and are now urging people to come forward.
Detective Chief Inspector Kate Kieran, a homicide detective from the Met’s specialist crime command, said: “It is incredibly sad and frustrating that Adam’s murder remains unsolved. The homicide command have been working tirelessly over the years to find out who is responsible.
“We recognise people may not have wanted to speak up at the time and may have felt loyal to the person or people involved in this.
“However, over the past 20 years, allegiances and relationships may have changed and some people may now feel more comfortable talking to us. We implore them be bold and come forward if they know something so that we can finally deliver justice once and for all.
“No matter how old or small that information may seem, it really could make all the difference.
“This young boy has not and will not be forgotten. He deserved better and we will not give up on him.”
Police say they believe “relationships and allegiances may have changed” and are specifically appealing to people “whose connection or association with someone has now ended”
To date, the investigation has seen checks carried out on all UK missing people and extensive inquiries in London, other parts of the UK and abroad, including South Africa, Holland, Germany and Nigeria.
Police have also used new forensic technologies but to no avail.
Despite high-profile appeals over the years, including an appeal by Nelson Mandela to all the African communities across the world, the case remains unsolved.
A woman was previously arrested on suspicion of murder but was later bailed and released with no further action.
A man was previously arrested and interviewed in connection with the possible trafficking of Adam into the UK. but was also bailed and released.
Another man was also arrested and interviewed on suspicion of alleged trafficking offences before bailed and released.
Anyone with information relating this case should contact police on 101, Tweet @MetCC or call Crimestoppers anonymously on 0800 555 111.