Online gambling sites have been urged to remove "unacceptable" adverts likely to appeal to children.
Regulators have written a letter to operators calling on them to advertise responsibly.
Under 18s and other vulnerable people should be protected from exploitation, the Gambling Commission, the Advertising Standards Authority, the Committee of Advertising Practice (CAP), and the Remote Gambling Association said.
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 ('under 18'); and, generally available to view ('freely accessible').
"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."
Regulators added that such adverts, which are likely to appeal to children, are "unacceptable".
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.
The Local Government Association (LGA) also backed the call, saying that 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."