The European Commission has revealed it intends to investigate how online companies such as social networks are using information about consumers' web habits.

It is thought the EC is particularly concerned about the growing use of deep packet inspection techniques that allow broadband providers to track online activity even if consumers have tried to delete tracking cookies set by the websites they visit.

According to Consumer affairs Commissioner Meglena Kuneva, the terms and conditions to which people agree in order to gain access to some commercial websites frequently breach privacy rules. Kuneva wants to introduce a blacklist of misleading terms similar to lists that exist for offline marketing companies.

"Trading your personal data in return for free web-based services is increasingly becoming the norm on which companies build their business models," said Helen Kearns, Kuneva's spokeswoman.

"The Commissioner wants to make sure that people are aware of this and that they aren't tricked into handing over more than they want to," she added.

Personal data is "the new petroleum of the internet world - a vital and valuable commodity. If you are happy trading your data that's fine, but you should at least know how valuable it is," she said.

Social networking sites including Facebook are among those raising concerns, Kearns said. Facebook recently sparked a storm of protest when it announced it was considering changing its terms of use to allow it to use information about people even after they had deleted it from their profile pages on the site. Facebook subsequently dropped the idea.

"It wasn't regulators who spotted the proposed change of terms at Facebook, it was one of the 175 million users," Kearns said.

"The Commissioner doesn't want to obstruct this advance in the online world. She recognises that it offers consumers fantastic opportunities. But she won't tolerate foot-dragging by companies when it comes to respecting people's personal data," Kearns said.