Facebook has been ordered to curb its data collection practices in Germany after a landmark ruling on Thursday that the world’s largest social network abused its market dominance to gather information about users without their consent.

Germany, where privacy concerns run deep, is in the forefront of a global backlash against Facebook, fuelled by last year’s Cambridge Analytica scandal in which tens of millions of Facebook profiles were harvested without their users’ consent.

The cartel office objected in particular to how Facebook pools data on people from third-party apps — including its own apps like WhatsApp and Instagram — and its online tracking of people who aren’t even members through Facebook “like” or “share” buttons.

“In future, Facebook will no longer be allowed to force its users to agree to the practically unrestricted collection and assigning of non-Facebook data to their Facebook accounts,” cartel office chief Andreas Mundt said.

Facebook said it would appeal the decision by the Federal Cartel Office, the culmination of a three-year probe, saying the anti-trust watchdog underestimated the competition it faced, and undermined Europe-wide privacy rules that took effect last year.

“We disagree with their conclusions and intend to appeal so that people in Germany continue to benefit fully from all our services,” Facebook said in a blog post.

In its order, the cartel office said it would only be allowed to assign data from WhatsApp or Instagram to Facebook if users consented voluntarily. Collecting data from third-party websites and assigning it to Facebook would similarly require consent.

If consent is withheld, Facebook will have to substantially restrict its collection and combining of data. It should develop proposals to do this within 12 months, subject to the outcome of appeal proceedings, the regulator said.

If Facebook fails to comply, the cartel office could impose fines of up to 10 per cent of the company’s annual global revenues, which grew by 37 percent to $55.8 billion last year.

German justice minister Katarina Barley welcomed the ruling. “Users are often unaware of this flow of data and cannot prevent it if they want to use the services,” she told Reuters. “We need to be rigorous in tackling the abuse of power that comes with data.”

The German anti-trust regulator’s powers were expanded in 2017 to include consumer protection in public interest cases where it could argue that a company — such as Facebook — had so little competition that consumers have no effective choice.

Facebook has an estimated 23 million daily active users in Germany, giving it a market share of 95 per cent, according to the Cartel Office which considers Google+ — a rival social network that is being closed down — “to be its only competitor. The next most popular social media sites in Germany are Pinterest and Google’s YouTube.

Facebook said the cartel office failed to recognise the extent of competition it faced from YouTube or Twitter for users’ attention, and also said the regulator was encroaching into areas that should be handled by data protection watchdogs. Brussels-based anti-trust lawyer Thomas Vinje of Clifford Chance said the decision had potentially far-reaching implications. “It’s limited to Germany but strikes me as exportable and might have a significant impact on Facebook's business model,” he said.

The European Commission said: “We are closely following the work of the Bundeskartellamt both in the framework of the European Competition Network and through direct contacts.”

“The European legislator has made sure that there is now a regulation in place that addresses this type of conduct, namely the General Data Protection Regulation,” it added. Facebook said it had rebuilt the information its provides people about their privacy.