A three-year-old girl and a boy, aged five, were critically injured in the fire in Preston on Friday.
Two children left critically injured in a house fire have died in hospital four days after the blaze.
The girl and boy, aged three and five, were rescued from the fire in Coronation Crescent, Preston, on Friday evening.
Their mother, aged in her 20s, was not seriously injured, Lancashire Police said.
A 24-year-old man held on suspicion of arson with intent to endanger life was later released with no further action.
Emergency services were called to the house at about 20:00 BST after receiving reports of people trapped inside the home.
A joint investigation between police and the fire service to find the cause of the fire is continuing.
Det Ch Insp Rach Higson said: “This tragic incident has resulted in two children sadly losing their lives and my thoughts are with their loved ones at this time.”
The children’s family are being supported by officers and a file will be passed to the coroner, police said.
