The UK’s advertising watchdog has upheld complaints against William Hill, Betfair, Dunder and LottoGo.
The Advertising Standards Authority found that the four firms failed to take appropriate measures to prevent children from seeing their ads when they appeared in the Looney Tunes World of Mayhem app in February.
The app gave players the opportunity to earn "gems" to use in the game by viewing ads or performing tasks.
The ASA ruled that the ads must not be used again without further specific targeting to minimize the likelihood of under-18s being exposed.
