Practices such as 'astroturfing', where employees pose as ordinary members of the public to post favourable reviews about their company, are banned under the new Consumer Protection from Unfair Trading Regulations.

Also banned is 'sock-puppeting', where a marketer poses a question online, then logs in as different people, all posting favourable responses. These practices have been punishable by a fine or imprisonment since the end of May.

'Most people don't know about this law,' said Gareth Thomas, director at Brands2Life. 'Also, there is a misconception that these devices are clever, but they can backfire.'

Drew Benvie, director at Hotwire, said: 'This law change affects everyone in PR. If customers have any presence online, it's definitely their business to know about it.'