More than 450 online gambling operators have been sent stern letters from UK regulators warning that they will be hit with sanctions if found guilty of targeting advertising at under-18s.

Officials from four regulatory bodies ordered the digital services to "amend or remove" advertising which could be considered appealing to children – including those displayed on paid and free-to-play games online.

"The use of particular colours, cartoon and comic book images, child and youth-orientated references and names of games such as "Piggy Payout", "Fluffy Favourites", "Pirate Princess" and "Jack and the Beanstalk" are likely to enhance appeal to under-18s," the joint letter read.

The fresh demands were sent by the Gambling Commission, alongside the Committee of Advertising Practice (CAP), Advertising Standards Authority (ASA) and Remote Gambling Association (RGA).

The move came after an investigation revealed that betting websites such as Paddy Power, 888 and Casinoland were luring children into gambling with cutesy images and games.

According to The Sunday Times, which conducted the probe, the Gambling Commission found that approximately 450,000 children gamble in England and Wales every week.

As a result, the team of regulators issued an urgent message to all websites in the industry to "act now".

They stated: "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.

"This includes affiliate ads that promote your company and its products.

"We appreciate that there may be difficult and nuanced decisions to be made, particularly in cases where imagery and characters, which might have appeal to both children and adults is used.

"If gambling operators cannot or will not bring their advertising into line with the [law], the CAP Compliance team has various sanctions available to it and will consider applying these if we continue to see gambling ads that have particular appeal to under-18s."

The letter did not elaborate on what sanctions could potentially be applied against rogue ads.

"Companies need to acknowledge the harm they are doing by cynically targeting children online and remove these games, many of which can be played for 'fun' and without age verification, from their sites," UK shadow secretary Tom Watson told The Sunday Times.

"This loophole is ruthlessly exploited [...] and it's one that urgently needs closing," he added.

It emerged this week that up to 95% of sports games broadcast in the UK on Sky Sports, BT Sport and ITV, prominently featured gambling services during commercial breaks.