Adverts for the bookmakers William Hill and Paddy Power Betfair appeared in a mobile phone app approved for seven-year-olds, featuring cartoon characters such as Bugs Bunny and Daffy Duck, in an apparent mistake that has been called “unacceptable”.

The Advertising Standards Authority upheld complaints after users of a game called Looney Tunes World of Mayhem were invited to view adverts for the betting companies in exchange for “gems”, tokens used to purchase in-game items.

In each case, users were asked to deposit a minimum of £10 in a gambling account in order to receive the gems.

“Given the use of cartoon characters, cartoonish violence and the relatively simple nature of the game, we considered it was likely to appeal to many under-18s,” the regulator said.

The ASA previously banned a gambling advert that appeared in an app linked to the ITV programme I’m A Celebrity Get Me Out of Here, after the Guardian reported concern about its potential impact on children from figures including the Labour deputy leader, Tom Watson.

Both William Hill and Paddy Power said the appearance of their adverts in the Looney Tunes game was the fault of Tapjoy, a US mobile advertising company.

Tapjoy told the ASA that it had accidentally labelled the game as suitable for “mature gambling” advertising and had now corrected the error, preventing it from appearing again.

It played down the appeal to children of the Looney Tunes game, in which players choose characters such as Porky Pig and Road Runner and battle each other using comic weapons.

“Tapjoy said that while adults of a certain age associated the Looney Tunes characters with their own childhood, the app in question used characters and entertainment properties dating back to the 1930s and 1940s,” the ASA said in its ruling.

The ASA said Tapjoy had also pointed to the Pegi 7 rating on the game, which indicated the “minimum” maturity level required to play the game, not necessarily the age group at which it was targeted.

Tapjoy also pointed out that its platform allows advertisers to focus their campaigns more narrowly by making use of data about users’ other preferences and self-reported demographic details, but that neither William Hill nor Paddy Power chose this option.

The company said it had a “longstanding policy prohibiting the use of Tapjoy with child users or apps directed toward children” and had corrected the mistake as soon as it was discovered.

But the Labour MP Carolyn Harris, a longtime campaigner on gambling, said: “I am deeply concerned that online gambling companies are marketing their products at children and enticing them to gamble. This is wholly unacceptable.”

Marc Etches, the chief executive of the UK’s leading gambling charity GambleAware, said: “The issues highlighted in this ASA ruling rightly underlines that it is unacceptable for children to be exposed to gambling online.”

Scopely, the publisher of Looney Tunes World of Mayhem, said the ad went against its advertising policies and such ads were strictly prohibited under its contracts with its advertising partners.

The company also told the ASA that it did not target games at children and was “not aware of any children playing Looney Tunes World of Mayhem.”

The ASA said the adverts must not appear again without more effective tools to ensure that they are not seen by children.