An investigation has begun into whether celebrities are making it clear when they are paid to endorse a product on social media.

The UK's competition watchdog has written to a number of influencers, including household names, to question how they operate.

These people, often with millions of social media followers, should make any commissioned posts immediately clear.

The regulator said it would name anyone found to be flouting the rules.

The Competition and Markets Authority (CMA) has not released the names of celebrities, thought to be in double figures, who have been contacted as yet, but a string of well-known people have been admonished by the UK's advertising watchdog in recent times.

The CMA said that celebrities could sway the shopping habits of millions of people by telling followers on channels such as Twitter, YouTube and Instagram where they have been on holiday, what they wear, what products they use or which books they read.

Consumer protection regulations state that anyone should make it clear in editorial content at first sight if they have been paid or rewarded in some way.

The risk is that consumers might be more likely to place their trust in a product if they think it has had the personal recommendation of someone they admire, without realising the post is part of a commercial agreement.

"It is really important they are clearly told whether a celebrity is promoting a product because they have bought it themselves, or because they have been paid or thanked in some way by the brand," said George Lusty, senior director at the CMA.

The CMA has asked the public to share their experiences as part of the investigation.

The investigation, which has no set deadline, could lead to naming and shaming of anyone breaking the rules, and potentially prosecutions. Courts can issue unlimited fines or even imprisonment in extreme cases.

Dozens of celebrities were warned about endorsing products on social media when the CMA took action against an intermediary in 2016.