(CNN) - Under a threat of potential jail time, a group of British online influencers including Ellie Goulding, Rita Ora and Zoella have agreed to change how they post online, after their social media profiles were investigated by an advertising watchdog.
Sixteen celebrities, with tens of millions of followers between them, were probed over whether they were posting "misleading" advertisements online -- an offense that breaches UK advertising law and carries a potential penalty of two years in prison.
The stars said they will now clearly state whether they have been paid to promote a brand or product, the UK's Competition and Markets Authority (CMA) said.
Models Alexa Chung, Rosie Huntington-Whiteley and Iskra Lawrence were among the group pledging to alter their posting habits, alongside a host of British reality TV stars.
Warning letters have also been sent to other, unnamed celebrities, noting that "concerns have been identified" about their posts and urging them to change their practices, 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," Andrea Coscelli, chief executive of the CMA, said in a statement.
"You should be able to tell as soon as you look at a post if there is some form of payment or reward involved, so you can decide whether something is really worth spending your hard-earned money on," he added.
Celebrities are regularly paid by brands to subtly advertise their products to their millions of followers, such as through product placement in photos.
But UK consumer protection law requires that they disclose whether they have been paid or incentivized to endorse a brand. The law that related to the investigation carries penalties of fines, up to two years in prison, or both.
Coscelli said the agreement "sends a clear message to all influencers, brands and businesses that they must be open and clear with their followers."
Goulding and Ora each have more than 14 million followers on Instagram, while Zoella, whose real name is Zoe Sugg, has racked up more than 1 billion views on YouTube.
The investigation did not make a finding about whether the celebrities have breached the law, the CMA noted.
But the group said they would be carrying out further investigative work into the "role and responsibilities of social media platforms."
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