The online gambling company Sky Betting & Gaming has been reprimanded by the data regulator for unlawfully sharing customers’ information with advertising companies that could then target those users with personalised marketing.

The Information Commissioner’s Office (ICO) said it had investigated Bonne Terre Ltd, trading as Sky Betting & Gaming, after a complaint by the campaign group Clean Up Gambling.

The group alleged that Sky Betting & Gaming was misusing its customers’ personal data to target vulnerable gamblers.

Sky Vegas, one of the company’s brands, has previously been fined £1.2m by the gambling regulator for sending “free spins” to people with a gambling problem during Safer Gambling Week.

On this occasion, the ICO said it had found no evidence to support the suggestion from Clean Up Gambling that Sky Betting & Gaming, part of the £30bn global gambling firm Flutter, had deliberately targeted vulnerable gamblers.

But it did find that the company had processed people’s data through the use of advertising cookies, without asking them, over a seven-week period between January and March 2023.

The online gambling company then shared customers’ data with advertising technology companies immediately, before they had been given the chance to accept or reject cookies.

Stephen Bonner, the deputy commissioner of the ICO, said: “We’ve all seen adverts online that seem designed specifically for us, such as an ad for trainers after signing up to a gym online. Some people may be happy to consent to receive these but others may not be comfortable receiving similar adverts, especially when it comes to sensitive aspects of our digital activity.

“For example, if you are visiting a gambling website or looking up concerning health symptoms, you may want to prevent this personal information being shared with advertisers.”

A spokesperson for Sky Betting & Gaming said it “welcomes the ICO’s determination that, after an exhaustive 18-month investigation, it found no evidence to support the claims levelled at us by campaign group Clean Up Gambling.

“We regret the accidental technical error which resulted in some customers’ information being incorrectly shared with our digital advertising partners without consent being obtained for a seven-week period at the beginning of 2023. We rectified this error within a day of becoming aware of it.”

Sky Betting & Gaming licenses the Sky brand from the broadcasting business of the same name but is not part of the same company. In 2018, the private equity group CVC and Sky Group sold their betting business for $4.7bn (£3.6bn) to The Stars Group, which merged with Flutter the next year.

The ICO has been on a drive to improve companies’ compliance with rules governing how internet users’ personal information is collected and sold on. Last year, the regulator reviewed the UK’s top 100 websites, including those of Sky Betting & Gaming, and discovered problems with how more than half were using advertising cookies.

It said the only one of 53 that had not complied with a request to improve its processes was the gossip website Tattle Life, which it said would now be investigated over its use of cookies and “apparent failure” to register with the regulator.

The Guardian has approached Tattle Life for comment.