The ASA monitored 24,000 Instagram ‘stories’ and found that a quarter of them were ads, but just 35% were clearly labelled as such.

So-called ‘influencers’ are paid by brands to promote companies and their products on social media accounts such as TikTok and Instagram.

The ASA published guidance in 2018 which set out the rules influencers are expected to stick to. Posts where influencers have been paid or received another benefit from a brand need to labelled as advertising or sponsored, or marked with a specific hashtag (for example, #ad).

However, the ASA has found unacceptable levels of non-compliance with the rules. The watchdog said it had given ‘fair warning’ to influencers as to what was expected of them and it would now look to name and shame the worst offenders.

The watchdog also warned that influencers should not rely on bios or past posts to make it clear to consumers they are connected to a product or brand.

Complaints to the ASA about influencers increased by 55% last year, from 1,979 to 3,144.

Guy Parker, ASA chief executive, said: “There’s simply no excuse not to make clear to the public when positive messages in posts have been paid-for by a brand. While some influencers have got their houses in order, our monitoring shows how much more there is to do. We’ve given influencers and brands fair warning. We’re now targeting our follow-up monitoring and preparing for enforcement action.

Neena Bhati, head of campaigns at Which?, said: “Social media users should be able to easily tell what is and what isn’t an ad on platforms like Instagram or TikTok. Influencers and the brands they work with should know the rules and must not muddy the waters between paid-for advertising and authentic endorsements.

“It’s good to see that the ASA is taking a proactive approach to dealing with influencers flouting the rules. The ASA must continue to keep track of this issue and work with other regulators to take strong action where necessary – sending a clear message that deceiving consumers for cash on social media will not be tolerated.”