Zoella, Rita Ora and Alexa Chung among influencers forced to declare sponsored posts more clearly

If the social media stars fail to comply they could face legal action leading to heavy fines or even prison sentences

As any social media addict will know, there’s nothing more frustrating than seeing someone post a glowing review of an item and wondering whether it really is as great as they claim, or they have a vested interest in selling it to you.

Now, 16 social media stars have formally agreed to be more transparent about the products they post about online, making it easier for the consumer to discern the reliability of endorsements.

Influencers including Zoella, Ellie Goulding and Alexa Chung have committed to clearly disclosing whether they have been paid to mention a product or service, and even whether it has been gifted to them for free.

The move follows an investigation by the Competition and Markets Authority (CMA), which has claimed that if the influencers fail to comply they could face legal action leading to heavy fines or even prison sentences.

Posts which have been paid for by brands will need to be clearly labelled with #ad or #spon, for example. The CMA suggests #freebie when posting about items which have not been paid for by the influencer, which is common during brand trips and events, or PO box "hauls" – both common ways in which brands can get a free mention from an influencer without paying for an ad.

Andrea Coscelli, chief executive of the CMA, said: “Influencers can have a huge impact on what their fans decide to buy. People could, quite rightly, feel misled if what they thought was a recommendation from someone they admired turns out to be a marketing ploy.”

This echoes the feeling among many viewers that the way influencers disclose sponsorships is unclear and often disingenuous. There are even YouTube channels dedicated to calling out influencers for their actions.

Earlier this month, the Advertising Standards Authority (ASA) announced it had cautioned "between 200 and 300" influencers for breaking the rules around paid-for posts on social media.

Further influencers who have agreed to the CMA guidelines include Jim Chapman, Binky Felstead, Rosie Huntington-Whiteley, Michelle Keegan and Rita Ora.