#ad: Tweeting celebrities warned to reveal commercial interests following Coronation Street Dispatches sting

Coronation Tweet: Brooke Vincent, one of the ITV soap actresses caught up in Channel 4's Dispatches sting (PA)

Celebrities who brazenly plug freebie products on Twitter have been warned to use the hashtag “ad” to avoid falling foul of the law.

A tweeted endorsement by a celebrity to their army of followers is now seen as the most effective form of direct advertising for many youth-focused brands.

Sponsored tweets don’t come cheap - socialite Kim Kardashian earns up to £6,300 for every message she posts on her account.

Revellation, a UK celebrity agency which works with some of the Coronation Street actresses who fell victim to this week’s Channel 4 Dispatches sting, boasts on its website: “Artists now available to tweet about your product or service.”

But celebrities are being advised to familiarise themselves with the European Union’s Unfair Commercial Practices Directive before firing off a “thanks xxx” tweet for their free luxury baby stroller.

Celebrities and advertising agencies must make it clear if there is a commercial relationship behind a product-endorsing tweet.

Robin Grant, global CEO of We Are Social, a fast-growing agency which specialises in digital marketing, said: “Celebrities are immensely powerful – people are more interested in celebrities than they are in brands. Involving celebrities in an ethical way can make a positive difference in a campaign.”

Mr Grant added: “It is illegal not to disclose if a person has been paid to promote a product but this consumer protection regulation has not yet been tested in the courts. What if a person has been given a service as a gift? The best way to disclose an interest is to include the hashtag ‘ad’ in the tweet.”

The Coronation Street sting will not deter the celebrity tweet industry. “The actresses may have been naïve but they haven’t done anything wrong, according to the Advertising Standards Authority (ASA) rules,” Mr Grant believes.

The actresses say they did not receive free gifts in return for tweeting and deny participating in any unlawful marketing promotion.

ASA guidelines state that sponsored messages on social networks should be clearly marked to avoid misleading followers. The ASA investigated footballer Rio Ferdinand and model Katie Price, who were paid to plug Snickers, after complaints that their tweets were not obviously identifiable as adverts.

The ASA dismissed the complaints because the tweet included the hashtag “spons”, indicating it had been sponsored, next to the address @SnickersUK.

However the advertising body banned a NikeTwitter campaign featuring the Arsenal star Jack Wilshere. He ended a tweet: “#makeitcount. gonike.me/Makeitcount” but had not disclosed his sponsorship deal within it. “If you’re paying somebody to send tweets, you have to make it clear,” said Mr Grant.