Police were called by the London Fire Brigade at approximately 5.56am on Saturday, 6 March to reports of a fire at a residential property in Russett Way, SE13.
Three people, a 34-year-old woman and two boys aged five and 14, sustained injuries and were taken to south London hospitals.
Despite the best efforts of the emergency services and hospital staff, the five-year-old boy sadly died shortly before 8am.
The 34-year-old woman and the 14-year-old boy remain in hospital in a serious condition.
Detectives and crime scene examiners are working closely with the London Fire Brigade to establish the cause of the fire.
The family of those involved have been informed and are being supported by specialist family liaison officers.
Detective Chief Inspector Laurence Smith, who is leading the investigation, said: “A child has lost his life in extremely tragic circumstances. At this stage, I am keeping an open mind as to the cause of the fire. I urge anyone with information to come forward.”
Anyone who knows anything that could help detectives should call the incident room on 0208 345 3985 with the reference Operation Tampere or CAD1321/06MAR.