Bianca Couzens was already dead when the fire was started, Scotland Yard said last night.
She was found by firefighters tackling a small blaze at the three-storey house in Leytonstone, east London, where she and her mother Susan, 31, had lived for about a month.
She was under bedding and clothing on a smouldering sofa, which firefighters moved into the garden. Her face was bruised. Officers are still trying to trace Mrs Couzens and fear for her safety. The child's body was formally identified by her divorced father.
A Scotland Yard spokeswoman said last night: 'We have launched a murder inquiry because we are satisfied that she was dead at the time the fire started.
'Although we do not yet have a cause of death, natural causes have been ruled out. The child was murdered.'
More tests were ordered after a post-mortem examination yesterday failed to explain how Bianca died. She and her mother were recently rehoused by the council from another house in Leytonstone.
Mrs Couzens is 5ft 5ins tall, with a medium build and short wavy hair and brown eyes. She usually wore a black track-suit.
Meanwhile, in North Wales a man died in a vain attempt to save his two young sons trapped by fire in their bedroom yesterday. Firemen brought out all three from the burning house at Holywell, Clwyd, but they died later in hospital.
They were named as Kenneth Davidson, 25, and his sons Jonathan, aged four, and Damien, two.
His wife Dawn, 23, and a year-old daughter, Jody, were being treated in hospital.Reuse content