Four young children were treated for carbon monoxide poisoning after their grandmother brought a barbecue indoors to dry her washing.
The woman, who has not been named, lit the device which she set up in the family kitchen before leaving the house.
Her three-year-old granddaughter collapsed when she was overcome by the deadly gas later that afternoon and was treated in hospital with five of her relatives.
They included two boys aged two and 10 months, a four-year-old girl and the woman's two daughters-in-law, aged 26 and 29.
Firefighters were called to the home on Hockley Avenue, in East Ham, east London, on Wednesday afternoon, soon after the grandmother set up her makeshift launderette.
A spokesman for London Fire Brigade said the elderly woman carried the barbecue in from her garden at around 3pm and placed it near the back door to speed up the drying of clothes which she hung out around the kitchen.
She then walked out, leaving her daughters-in-law in the house with her grandchildren, two of whom were asleep upstairs.
One adult began to feel unwell before the little girl collapsed.
All six have now been discharged from hospital.
Fire chiefs branded the grandmother's behaviour dangerous and campaigners warned of the hazards of bringing barbecues indoors.
Dave Brown, of London Fire Brigade, said: "In my 28-year career I have never heard of anybody using a barbecue to dry clothes let alone using one indoors.
"Never, ever bring a lit or smouldering barbecue indoors. Not only is it a serious fire risk but it also omits carbon monoxide (CO) which is a poisonous gas that can kill or seriously injure."
Christine McGourty, of the Carbon Monoxide - Be Alarmed! campaign, said: "Using a barbecue indoors is always dangerous, but carbon monoxide poisoning is usually caused by faulty or poorly maintained fuel appliances, such as boilers, ovens and fires. We'd urge everyone to get an alarm and make sure their family is safe."