ITV has come under fire for allowing a gambling company to sponsor the I’m a Celebrity … Get Me Out of Here app, amid concern that hundreds of thousands of child fans of the show are being “bombarded” with encouragements to bet.

Labour’s deputy leader Tom Watson said it was “obviously wrong” that a gambling company should sponsor an app popular with under-18s, in the light of a report by the Gambling Commission that found the number of children who are problem gamblers has risen sharply to 55,000.

The app, which has been downloaded more than 1m times, is the most popular method by which viewers can vote on I’m a Celebrity, the most watched TV show of 2018 outside the World Cup with a peak audience of 11.9m.

One media agency estimated the number of under-18 viewers per episode at more than 1 million during the last series.

It is sponsored by Tombola – an online bingo, casino and slots company – and the app displays adverts featuring phrases such as “A chance to win a share of £250,000 for free” when users sign up to vote on the TV show.

Users who download the app can click through to casino-style “slots” games and other forms of gambling.

Labour’s deputy leader Tom Watson said: “It is obviously wrong that non-age-verified apps can bombard young people, who are simply trying to vote on a TV show, with gambling adverts.

“The ads are near impossible to navigate around and make gambling seem risk-free and fun.”

He added that the adverts were “unacceptable” in light of the recent rise in the number of young people with gambling problems.

“Operators, broadcasters and the government need to wake up to this crisis and start acting more responsibly,” he said.

“They must ensure adverts on social media cannot be targeted at young and vulnerable people.”

Joanna Murphy, who uses Twitter as Paloma de Paz, said her 13-year-old son had downloaded the app and believed he had to sign up to Tombola’s gambling website to be allowed to vote on I’m a Celebrity.

“Tombola sponsoring it prompted my son to ask about gambling apps in general and the ‘free money’ offers but he has good understanding of the principle that if it seems to good to be true it usually isn’t,” she told the Guardian.

“Other kids might not and could easily be pulled into it.”

The situation highlighted the possible influence these adverts can have on young people, she added.

“It’s a family thing for us to all watch it together and it was rather a shock to get questions about gambling instead of the usual ones about [previous app sponsor] Iceland.”

Marc Etches, chief executive of GambleAware, said: “Frankly, it is completely unacceptable that children are exposed to gambling in this way. The combination of gambling marketing and a blurring of the lines between computer games and gambling are all contributing to the normalisation of gambling for children.

“It is shocking that 55,000 11- to 16-year-olds are problem gamblers. Gambling has become a public health issue in Britain and all of us, including broadcasters, share the responsibility to prevent gambling-related harms generally, and to protect children especially.”

ITV responded: “The I’m a Celebrity sponsorship is fully compliant with Ofcom rules on programme sponsorship and the broadcast committee on advertising practice (BCAP) code rules on content and scheduling.”

I’m A Celebrity is not specifically targeted at children and begins after the 9pm watershed.

However, a media agency told the Guardian that the show was estimated to attract 1 million young viewers per episode last year, nearly a tenth of its audience.

The Advertising Standards Authority has previously upheld a complaint against a mobile phone app, targeted at all ages, because it featured a gambling advert seen by children under 10.