Lord Alan Sugar has been made to remove a tweet promoting a teeth whitening product after breaching advertising guidelines.
The Apprentice host, 73, said the Stylsmile Teeth Whitening Toothbrush Kit would be a "perfect Xmas gift" but did not make clear that the post was an ad, the Advertising Standards Authority (ASA) said.
Lord Sugar is a director of the manufacturer Stylideas Ltd and owns a 50% share in the business – sharing it with a previous winner of his popular TV show.
"If you know someone who's longing for whiter teeth, this is the perfect xmas gift for them," he tweeted on December 9 last year, including a link to the product web page.
The ASA said Lord Sugar's connection to the company was well-known as it had been made public on a prime-time show in front of millions of viewers.
Marketing guidelines say advertising communications must be obviously identifiable as such, and must make clear their commercial intent if it is not obvious from the context.
Posts should, at the least, include the hashtag #ad.
The ASA said Lord Sugar's tweet had not made his commercial interest in the company "immediately clear" to consumers.
It ruled that the ad must not appear again in the form complained of, adding that future posts by Lord Sugar and Stylsmile should make all commercial intents in their posts clear.