Gambling websites have been ordered to immediately remove "unacceptable" adverts likely to appeal to children.
Regulators have jointly written to operators urging them to advertise responsibly and threatening them if sanctions if they fail to comply.
The highlighted games with names such as Fluffy Favourites and Pirate Princess, which featured brightly coloured cartoon characters, as examples likely to lure children into gambling.
Under-18s and other vulnerable people should be protected from exploitation, the regulators stressed.
The letter said: "We are writing to advise you to amend or remove immediately any ads on your website or in third party media that are likely to appeal particularly to people aged 17 or younger… and generally available to view.
“This relates particularly to freely accessible ads for play-for-free and play-for-money games and includes all graphics and images displayed on a website or in third party media.
“The use of particular colours, cartoons 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.”
It added: "You must immediately amend or remove any freely accessible ads on your website or in third party media space that are likely to appeal particularly to under-18s."
The letter warned of possible sanctions should sites fail to comply with the CAP code, which requires marketing communications for gambling to be socially responsible.
About 450,000 children gamble on a weekly basis, research by the Gambling Commission found last year. About 9,000 of those were described as "problem gamblers".
The Local Government Association (LGA) backed the crackdown, saying councils had previously asked for greater restrictions.
Councillor Simon Blackburn, chairman of the LGA's Safer and Stronger Communities Board, said: "Problem gambling is a major concern for councils which can cause greater personal harm.
"It can lead to spiralling debt, deteriorating mental health and wellbeing, and a toll on society - and taxpayers - through crime and disorder, family breakdown and homelessness.
"It is vital our children and young people are kept safe and protected from the problems gambling can cause."
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