Olympic athletes were slapped with a warning from the UK's Office of Fair Trading today after a rash of thinly veiled advertisements were noticed coming from their Twitter accounts.

The Daily Mail reports that British sports stars have been raving over everything from cars to cosmetics and chocolate on the popular social network, increasingly more so as the 2012 Olympics draw near.

The watchdog group's problem is not with the promotional messages themselves, but with a lack of transparency among athletes -- some of whom they say were paid large amounts of money in exchange for shout outs.

"Practically speaking... the tweet or Facebook post involved should state clearly that the celebrity is being paid for the endorsement, eg sponsored," said an OfT spokesperson.

Athletic sponsorship is far from a new phenomenon, but social networks have only just entered the debate and the OfT maintains that athletes should declare if they have a financial link to a brand when naming them on Twitter.

Ladies singles champion Joannie Rochette has been promoting her sponsors to great effect using social media.

Rochette, who has over 35,000 followers on Twitter and 112,369 fans on Facebook, teamed up with her sponsor Lasik MD to run a successful social media campaign earlier this year in which her fans were encouraged to enter a contest via Facebook for a chance to watch a live Stars on Ice performance in Toronto. 

Lasik MD is one of Rochette's many sponsors, and is clearly stated as such on her website, fitting well within the OfT's stated guidelines.

Likewise, Canadian BMX racer Tory Nyhaug regularly retweets Coca Cola, but lists the company along with his other sponsors while shouting them out on Twitter.