Police found a two-year-old child in a room littered with drug needles.
Blood from people injecting themselves stained the floor and walls.
Officers were called to the address near Brighton seafront after complaints from neighbours.
They found the young girl surrounded in "squalor", according to one source.
The source said: "Put it this way, she won't ever be living there again."
Officers removed the child under a police protection order and she is now being cared for by foster parents.
The girl's 23-year-old mother was arrested and later released pending further inquiries.
The mother and child were apparently staying temporarily at the address when police called there on Sunday.
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