Met Police officers are investigating after being called to assist firefighters in Tillett Close, Stonebridge, near Wembley, shortly after 1.20am today.
A 43-year-old woman and three boys, aged 15, eight and four, died at the scene, the force said.
Two others were taken to hospital. Police said they are awaiting an update on their conditions.
A 41-year-old man was arrested at the scene in connection with the incident and remains in custody.
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