Rio, Katie and the Twitter Snickers adverts that left Mars feeling bitter

ASA investigates company after Rio Ferdinand and Katie Price promote snack bar on social network site

A ploy by the confectionery giant Mars to use sports and entertainment stars to promote the Snickers chocolate bar brand on Twitter is being investigated by the Advertising Standards Authority.

In a co-ordinated campaign, Snickers paid Rio Ferdinand, Amir Khan, Ian Botham, Katie Price and the X Factor contestant Cher Lloyd to post "twitpics" of themselves with Snickers bars to coincide with a television advertising campaign Mars is running.

Marketers have become increasingly desperate to reach the huge audience on Twitter, a rapidly growing media platform which does not carry banner advertising. Ferdinand, one of Twitter's most fervent users, has amassed nearly two million followers, a larger audience than some prime-time television shows. Price has more than 1.5 million followers, Lloyd has 900,000, Khan has more than 460,000 and @BeefyBotham has attracted more than 45,000 adherents. By contrast, many corporate brands have set up accounts but struggled to attract interest. Snickers UK boasted only 825 followers last night.

Mars has claimed its use of celebrity tweeting is within the rules and that it sent out a later tweet to make clear the stars were involved in a promotion.

But the ASA said yesterday that it had received complaints in relation to Ferdinand and Price and had launched a formal investigation into Mars. It is investigating whether the company should have labelled the initial tweets as marketing communications and whether the "reveal" tweets which were sent 90 minutes later to explain that the pictures were advertising were sufficiently clearly labelled for followers to understand.

Under Office of Fair Trading guidelines, advertisers are required to be transparent when promoting products via social media. But advertising on Twitter remains a grey area. Stars frequently use the platform to promote their products.

But some users feel such messages are against the spirit of the site. After Ferdinand posed with a Snickers bar, some of his followers tweeted him back. "Do you really need the money that badly?" asked one, while another told the footballer: "I'm not here to be advertised at."