The Office of Fair Trading is clamping down on celebrities that are paid to endorse brands on Twitter without making the relationship clear.

Following an investigation by the watchdog into commercial blogging network, Handpicked Media, the OFT is now insisting it must be clearly stated when promotional comments have been paid for.

The OFT said online advertising and marketing practices that do not disclose they include paid-for promotions are deceptive under fair trading laws.

"This includes comments about services and products on website blogs and microblogs such as Twitter," it added.

The practice of endorsing brands on micro-blogging service Twitter is common in the US, where celebrities including rapper Snoop Dogg, actress Lindsey Lohan and Kim Kardashian earn thousands of dollars for posting tweets featuring endorsements. However, the US Federal Trade Commission insists such tweets include the wards 'ad' or 'spon' to indicate the endorsement has been paid for. No similar requirement exists in the UK at present.

"The integrity of information published online is crucial so that people can make informed decisions on how to spend their money," added Heather Clayton, senior director of OFT's Consumer Group.

"We expect online advertising and marketing campaigns to be transparent so consumers can clearly tell when blogs, posts and microblogs have been published in return for payment or payment in kind. We expect this to include promotions for products and services as well as editorial content."