Fire crews attended a property in Barking Road, Upton Park, Newham, after two adults and two children went to hospital complaining about a smell of chemicals in a flat.
One of the children later died in hospital, the London Fire Brigade (LFB) said.
The Metropolitan Police is investigating Tuesday’s incident, “supported by the brigade”, LFB said.
Around 12 people were evacuated from surrounding properties as a precaution whilst crews responded.
Specially trained firefighters carried out a “systematic sweep” of the affected property, working to detect any hazardous materials present and ventilate the scene, LFB said.
In a statement, it said: “The first call reporting the incident was received at 1.18pm, and firefighters from Poplar, Millwall, Euston and surrounding fire stations attended the scene. The incident was over for crews by 4.23pm.”
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