Celebrities have been warned over misleading Instagram posts that are not clearly marked as promotions after complaints that scores of high profile users could be breaching consumer rights.

The US and UK are clamping down on the way influential users, who often boast tens of millions of followers, share paid-for material on the Facebook-owned social network.

On Wednesday night the Federal Trade Commission issued warning letters to 90 celebrities urging them to disclose when posts are sponsored clearly and high up in the message.

The UK watchdog, the Advertising Standards Agency, confirmed that it has also been cracking down on the phenomenon, working with companies and high profile users to ensure guidelines are met.

The most recent warning in the US comes after rights organisation Public Citizen raised concerns with the watchdog about the endorsement of 113 celebrities, including Rihanna, Rita Ora and Kylie Jenner.