Celebrity Fortnite players include England footballer Dele Alli, who has boasted of winning over 100 games. Picture: Flickr.com

London - The parents of a nine-year-old girl who became addicted to playing online video game Fortnite told of their horror at learning she had been secretly staying up all night on her Xbox.

As her mother and father slept, the primary school pupil became so immersed in the violent online world she would wet herself rather than take a break to go to the toilet.

In a stark illustration of how the hugely popular kill-or-be-killed game is affecting children, her parents said concerned teachers raised the alarm after their daughter began falling asleep in class.

The family enlisted the help of an addiction expert, who told the Mail he believed many more parents were unaware that their children were hooked on gaming right under their noses.

"Young children's brains are being pitted against the most compulsive game that Silicon Valley can put together, and it's no contest," said Steve Pope.

"Fortnite creates a mental itch that constantly needs scratching, it becomes a compulsion. I believe we've got a secret epidemic out there."

Celebrity Fortnite players include England footballer Dele Alli, who has boasted of winning over 100 games. England manager Gareth Southgate said he expects younger members of his World Cup squad to use their downtime in Russia playing "Fortnite or whatever it is".

The parents of the previously sporty nine-year-old bought her the Xbox in January, and within two months noticed a change in her behaviour.

READ: The darker side to the online game that's becoming a global phenomenon

"We got called in by her headteacher asking if everything was okay," her mother Carol, 36, told the Sunday People.

"She had fallen asleep twice in lessons and her grades were slipping. When we asked our daughter what the problem was, she became unusually argumentative and aggressive, which we just put down to her hormones."

Furious, the parents confiscated the console - only for her to lash out with it and strike her father Richard. They then limited her to an hour a day of Fortnite on school nights and two hours a day at weekends - only to find her up in the early hours sitting on a urine-soaked cushion playing her Xbox.

"She was so hooked on the game she wouldn't even go to the toilet," her mother said. The girl confessed she had been waiting for them to go to sleep, then playing Fortnite until as late as 5am.

The parents enlisted Pope after their daughter became "withdrawn, agitated and disturbed".

She is now receiving help in overcoming her addiction and getting her life back to normal.