Lord Alan Sugar has been told to remove a tweet promoting a teeth whitening kit after he did not make it clear it was an advert.
The Apprentice host tweeted in December that a product from the brand Stylsmile would make a “perfect Xmas gift”.
He owns a 50% share in the business, which is run by a former winner of the reality show.
The advertising watchdog said the tweet breached guidelines and must not appear again.
“If you know someone who’s longing for whiter teeth, this is the perfect Xmas gift for them,” said Lord Sugar, 73, in the tweet, along with a link to the product’s website.
A complaint was raised about the post, questioning whether it was obvious it was an advert.
Lord Sugar and Stylsmile – run by inventor Tom Pellereau, who won the series in 2011 – argued that it was a well-known fact that Lord Sugar was a partner in the company, because it was made public on The Apprentice in front of millions of viewers.
They said Lord Sugar was known to post about his businesses on social media regularly, and the tweet was not a covert promotion.
However, the Advertising Standards Authority ruled that the tweet broke the advertising code. According to the rules, all advertising communications must be clearly marked, for example with the phrase “#ad” on social media posts.
The ASA said the tweet “was not obviously identifiable as a marketing communication”.
And although Lord Sugar was a well-known investor, it was also not immediately clear to people that Lord Sugar had a commercial interest in that business, the ASA said.
“We told Stylideas Ltd t/a Stylsmile UK and Lord Sugar to ensure that they made clear the commercial intent of their posts in future, for example by including a clear and prominent identifier on their social media posts such as #ad,” the watchdog added.
The ASA has also upheld complaints against TV personality Stacey Solomon, who posted two paid-for posts on her Instagram account promoting the brand Card Factory.
Her two Instagram stories, in November last year, contained the word “ad” but it was in white lettering on a white background.
The ASA said because the word was obscured, “the story posts were not obviously identifiable as marketing communications”.
Solomon is the latest among a string of celebrities and influencers to have had complaints upheld against them over social media posts that had not been marked clearly as adverts.