The girl's mum Carole, 36, told the Sunday People: "We had no idea, when we let her play the game, of the addictive nature or the impact it could have on her mental health".
Fortnite is the latest fixation for gamers - with more than 40 million people across the world downloading it since it launched a year ago.
In a bid to wean her off Fortnite, the couple limited their daughter's game play to one hour on weekdays and two hours on weekends, but she refused to follow the rules.
She would secretly play the survival shooter game during the night and didn't even get up to use the toilet because she couldn't prise herself away from the screen, the Daily Mail reports.
If you needed any further proof that Fortnite is coming to the Nintendo Switch, then here you go.
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Later on, the girl became withdrawn and disinterested in prior hobbies, like going to the gym and taking ballet classes. A couple from the United Kingdom sent their 9-year-old daughter to rehab after she became addicted to "Fortnite".
"This is a serious issue which is destroying our little girl's life and someone needs to step in to ban it before it becomes an epidemic". After a series of $67 charges were placed on the family's credit card by the girl for premium in-game items, her father went to confiscate the Xbox and was struck in the face by the out-of-control kid. "She had fallen asleep twice in lessons, and her grades were slipping", Carol said.
Nintendo announced the news on Tuesday morning during its annual briefing at E3, the video game industry trade show held in Los Angeles.
Addictions counsellor Steve Pope, who treated the nine-year-old, said he had been contacted by parents of children as young as eight about possible Fortnite addictions.
And professional gamers are making up to £500,000 (NZ$953,000) a month from live-streaming their matches for addicted children to watch.