A coroner has recorded open verdicts on the 13 black youngsters who died in the New Cross fire in January 1981.
Some believed arsonists were behind the blaze
The victims, aged between 14 and 22, died after flames engulfed a birthday party in south-east London.
The original inquests returned open verdicts but police began a new investigation in 1997 and a new inquest was ordered in 2002.
There had been speculation in the immediate aftermath of the fire that it could be the work of racists.
However, new forensic evidence led to that theory being largely discounted by the families of those who died and the survivors.
On Thursday, Gerald Butler, QC, recorded open verdicts at Southwark Crown Court.
"I have concluded on the totality of the evidence that while I think it probable, that is to say more likely than not, that this fire was begun by deliberate application of a flame to the armchair near to the television...I cannot be sure of this.
"It must follow I am unable to return a verdict of unlawful killing," Mr Butler said.
The families and survivors, who fought a long campaign for a second inquest, had hoped for a more decisive ruling.
As Mr Butler made his statement some relatives in the court gasped and muttered "no" in disbelief.
During his summing up the coroner said the first inquest had been held "too soon after the event", and emotions at the time were running understandably high and could have clouded the accuracy of witness statements.
"In 1981, many in the black community, particularly the young, were distrustful of the police and did not show that degree of co-operation that has been shown since the fresh inquiry into the fire began," Mr Butler said.
'Long way to go'
After the verdict, Labour MP for Lewisham Deptford Joan Ruddock, who has campaigned with the families for a proper investigation into the tragedy, said the verdict was "extremely disappointing".
George Francis' 17-year-old son died in the fire
Ms Ruddock, who was in tears at the end of the hearing, said: "We had hoped for a verdict of unlawful killing but we have been taken some way forward."
George Francis, whose 17-year-old son Gerry was killed in the fire, said his fight for justice was not over.
"I think we have a long way to go. We have not lost anything, in other words we have gained.
"For the simple reason we now know what really happened on that fatal night which we did not know at the 1981 inquest," Mr Francis said.
Outside the court Imran Khan, the solicitor for Armza Ruddock, whose daughter Yvonne and son Paul both died in the fire, read out a statement.
"Whilst legally the coroner appeared to have no choice but to return an open verdict, he has come as close as possible to say what I always believed, that the fire was started deliberately, albeit not by a petrol bomb," Mr Khan said.
Patrick Allen, the solicitor representing the families of the victims, said: "The families have waited many long years for justice to be done. The original inquest was a travesty of justice.
Family representatives outside Southwark Crown Court
"Justice delayed is justice denied. The fact that this inquest took 23 years has quite definitely caused prejudice to the search for the truth."
Mr Francis said the families' lawyers would consider whether there could be a review of the judgement.
Commander Steve Allen, head of the Metropolitan Police's Racial and Violent Crime taskforce which carried out the investigation, said the inquiry would never be regarded as "closed".
Mr Allen said he shared the view of the coroner that, on the balance of probabilities, the blaze had been started deliberately.
"If any new significant evidence or lines of inquiry came into our possession we would of course pursue them with vigour," he added.