The boy, who had a mental age of two and attended a special school, died with his family at their house in Leicester.

The inquest recorded a verdict of accidental death for all of the victims.

Several witnesses said Malcolm had a "fascination" with cigarette lighters and sticks and had previously started fires at his home.

Fire investigator, Station Officer Mike Murphy, said the most probable explanation pointed to the boy, although there was no direct evidence.

After previous incidents, in which he set fire to a quilt covering his mother and set a carpet alight, all lighters were kept in a padlocked box and no matches were allowed in the house, the coroner was told.

The heat was so intense that plaster fell from the walls

Firefighters found the bodies of Malcolm, his nine-year-old sister Kirsty, and their mother, 28-year-old Teresa Harcourt, in the burnt-out semi-detached council house on 22 January.

Malcolm's younger sisters, Kelly, three, and Kimberley, two, suffered severe burns in the fire and died later in hospital.

Ms Harcourt's boyfriend, Anthony Taylor - who had moved in three weeks before - told the hearing that Malcolm woke them up in the early hours, to tell them there was a fire.

Mr Taylor said he had jumped out of the upstairs window and waited for Teresa to throw her children down to him - but no-one had appeared.

Thick smoke

He had then tried to get back into the house through the front door but was beaten back by thick smoke.

The families of the children were allowed by the coroner to put questions to Mr Taylor.

He was asked why he had not raised the alarm sooner, but said he could not answer.

Leicester and South Leicestershire Coroner Martin Symington was told that neighbours heard the children crying out for help.

Pathologist Dr Clive Bouch said all the family were severely burned, but had died of smoke inhalation before the flames took hold.

Fire 'incredibly fast'

Mr Murphy told the inquest the fire may have begun slowly, but once it spread to the lounge furniture there would have been an "incredibly fast" development of fire and highly toxic gases.

The inquest was told a gas fire had been left on in the lounge and that it could not be ruled out as a cause although it was unlikely.

He also thought it was "theoretically" possible that the fire was started by a cigarette left by an adult.

It was not known how the fire started but an empty box of matches was found among the debris, a lighter was discovered in Teresa Harcourt's bedroom, and the child could have stuck something into the gas fire, Mr Murphy said.

Detective Sergeant Andrew Ross said the case was not suspicious and had been closed.

Alarm 'could have saved them'

The coroner said the public could only imagine the horror of the family members and emergency services who had witnessed the tragedy.

He offered "condolences to the families for the appalling tragedy.

"It was indeed a dreadful end to such young lives."

Outside the court, Station Officer Murphy said a smoke alarm could have saved the victims.

"This was one of the hardest cases we have dealt with. It was horrific," he said.

"Whatever the ignition source, a smoke alarm would have given more warning."