A family and their dog were trapped in their flat after arsonists set fire to the pet's cushion.
Firefighters believe burning materials were pushed through the letterbox of a first-floor flat in Lancing Court, Whitehawk, Brighton.
The dog's cushion in the hallway caught fire and the flames spread to the rest of the flat.
Two adults and an 11-year-old girl tried to escape from a balcony but had to be helped down a ladder to safety by fire crews.
Firefighters from Roedean and Brighton were called to the fire shortly after 9pm yesterday.
Leading Firefighter Mark Fisher, from Roedean, said the residents were shocked and had inhaled smoke but did not need hospital treatment.
He said that if the flat had a smoke detector, the family would have noticed the blaze sooner.
The hallway was scorched by the flames and the rest of the smoke suffered severe smoke damage.
The family decided to stay elsewhere overnight.
Fire crews remained on the scene until 10.30pm making sure the fire was completely put out.
The fire brigade and Sussex Police are investigating the cause of the blaze.
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