Regulators said the "unacceptable" ads and third-party media, which contain graphics and images that are likely to be attractive to minors, should be "immediately" removed or amended.

They said this particularly applied to adverts for free and pay-to-play games.

In a joint letter, the regulators said under-18s and other vulnerable people should be protected from exploitation.

The Gambling Commission, the Advertising Standards Authority, the Committee of Advertising Practice, and the Remote Gambling Association said companies needed to advertise responsibly.

Under the Committee of Advertising Practice (CAP) code, gambling activities must not be advertised in a way that could appeal to the under-18s.

Peter Pan

"The use of particular colours, cartoon and comic book images, animals, child- and youth-orientated references and names of games such as 'Piggy Payout', 'Fluffy Favourites', 'Pirate Princess' and 'Jack and the Beanstalk' are likely, alone or in combination, to enhance appeal to under 18s," the letter sent to more than 450 online gambling operators says.

The letter warned that those who failed to adhere to Advertising Standards Authority guidelines could face sanctions.

It follows an investigation by the Sunday Times that found gambling sites were using cartoons and characters including Peter Pan and Jack and the Beanstalk.

The paper said the letter was sent to more than 450 online gambling operators.

Tim Miller, executive director at the Gambling Commission, said new technology brought "opportunities but also new risks".

In a letter to the Sunday Times he said: "Our rules require strong age-verification checks to prevent underage gambling. Where businesses fail to protect vulnerable people, especially children, we have and will continue to take firm action."