Internet users must actively consent to being tracked around the internet with cookies, the European Court of Justice has found.

The ruling has the potentially to fundamentally change how people use the web, and the degree to which websites are allowed to follow their users.

In the major ruling, Europe's highest court said that websites can only track their users if they give explicit and active consent. A pre-ticked checkbox assenting to the tracking is not enough, judges said – websites must instead leave those boxes empty and allow users to tick them if they wish.

The ruling stems from a 2013 case when the German Federation of Consumer Organizations took legal action against online lottery company Planet49, which had a pre-ticked checkbox to authorise the use of cookies.

The cookies – data sent from a website and stored on a user's computer – collected information to help target advertisements for products offered by Planet49's partners.

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The consumer organisation argued this was illegal because the authorisation did not involve explicit consent from the user.

The German Federal Court of Justice asked for guidance from the EU's highest court to rule on the case in relation to EU laws on internet privacy. The EU court sided with the German consumer group, saying EU law aimed to protect consumers from interference with their private lives.

"A pre-ticked check box is therefore insufficient," the court said in a press release, adding that cookie consent must be specific and explicit and that clicking a button to participate in a game or browsing a website, and through that allowing cookies, was not enough.

The Norwegian Research Center for Computers and Law at the University of Oslo said in a statement that the ruling is "likely to have a significant impact on the ongoing negotiations on the ePrivacy regulation which is set to regulate cookie usage."

Several of the largest internet companies, such as Facebook and Twitter currently have implicit cookie consent, where by using the site, consent is deemed to have been given.

Facebook and Twitter were not immediately available for comment.

The case predates General Data Protection Regulation (GDPR) - the May 2018 internet privacy regulations that stipulate how companies must inform users about how their personal information is gathered.

The EU court also ruled that service providers had to fully inform users, including how long the cookies would operate for and whether third parties would have access to gathered data.