Gambling advertisements featuring Formula 1 racing driver Lewis Hamilton and the Chelsea football club logo have been banned in the UK over concerns that they would influence children.

The adverts published by two gambling firms, Kwiff and Betway, were banned after investigations into the complaints, the UK's Advertising Standards Authority (ASA) said on Wednesday.

Betway's YouTube advert, seen in May, featured football fans wearing clothes with the Chelsea badge, while Kwiff's post on X in July promoted the British Grand Prix with Hamilton's picture.

Both betting companies were warned not to include any character who had a strong appeal to viewers under the age of 18.

Betway told the BBC that it has fully cooperated with ASA's investigation and will implement its recommendations. Chelsea Football Club also said that it will work with partners to make sure its ads comply with the rules.

The BBC has contacted Kwiff for comments, as well as Mr Hamilton's team.

In its ruling on Kwiff, which is run by Eaton Gate Gaming, ASA said that a researcher from the University of Bristol had lodged a complaint over concerns that the firm's post on X would likely appeal to under-18s.

Kwiff's post featured an image of Hamilton with text highlighting a "huge weekend" for him at the British Grand Prix in the Silverstone race, accompanied by an "18+" symbol and the BeGambleAware.org logo, ASA said.

The post also included a link to an article on Kwiff's website about the race, it added.

Eaton Gate Gaming had said that its data indicated that Hamilton appealed to an older audience rather than those aged under 18, ASA wrote.

The post was meant to drive traffic to their their company blog, which was an "editorial commentary" on a website separate from their gambling platform, Kwiff told ASA.

Kwiff has since reviewed its social media accounts and removed any content that displayed mainstream sportspeople, ASA said.

"We considered Sir Lewis Hamilton was a notable star within the sport, with a significant public profile and social media following," the ASA ruling said.

The authority also cited Hamilton's 150,000 Instagram followers who were under 18 and based in the UK as a sign of his appeal among youths. It also said Hamilton had appeared in the F1 24 video game - which had an age rating of three years old and above - and was a storyteller on a programme aired by the BBC's CBeebies last year.

"As such, we considered that Kwiff would have been aware of the possibility that Sir Lewis Hamilton would have strong appeal to under-18s," it added.

Separately, ASA also ruled that Betway's advertisement breached its code.

YouTube, according to ASA, was an unsuitable platform for the advert, as advertisers could not guarantee that their content would exclude under-18 viewers.

"It was likely that there was at least a significant number of children who had not used their real date of birth when signing up to YouTube," ASA said.

Betway told ASA it had the contractual right to use Chelsea's logo in its role as the club's official European betting partner.

Betway said that YouTube's own ad policies offered further safeguards against under-18s being exposed to age-restricted content, according to ASA's statement.

Banning the advert could set a "damaging precedent for gambling sponsorships in sport", Betway told the report.

A spokesperson for the firm said it had no intention of publishing content that breaches industry rules and that its work goes through rigorous reviews before being released.

"This particular video was subject to the same thorough checks and was believed to be fully compliant with the latest industry guidelines," said the Betway spokesman.

A spokesperson for Chelsea said it will work with all its partners to ensure it complies with standards and practices across the industry.

"Our collaboration with Betway was no exception, and both parties believed the content of this advertisement was fully compliant and adhered to all guidelines."