Meta Platforms, owner of Facebook, on Tuesday criticised a landmark German antitrust order to curb its data collection as 'clearly flawed' and which undermines EU data protection rules.

Meta's criticism of the German antitrust watchdog came after the latter in 2019 said the world's largest social network had abused its market power by collecting users' data without their consent and ordered it to stop.

The competition enforcer said the data harvest included users' browsing habits when they visited a website with a Facebook 'like' button on it - even if an internet surfer didn't click on that button.

The case underscores the mounting regulatory scrutiny worldwide of U.S. tech giants and measures to rein in their power.

In the German case, however, the issue is also whether the watchdog exceeded its authority by using its antitrust power to address data protection concerns.

Meta challenged the decision at a German court which subsequently sought guidance from the Court of Justice of the European Union (CJEU).

The German antitrust order was "clearly flawed" with its "far reaching restriction on Facebook's data processing", Meta lawyer Hans-Georg Kamann told the panel of 15 judges.

He criticised the German watchdog for not cooperating with the Irish data protection regulator which supervises Facebook because its European headquarters is in Ireland.

"The Bundeskartellamt has openly undermined the substantive and procedural requirements of GDPR (General Data Protection Regulation) law," Kamann said.

Joerg Nothdurft, lawyer for the German watchdog, dismissed the criticism, saying there had been contact with data protection regulators.

The German government defended the antitrust decision, saying that it was imperative for the competition enforcer to make data protection assessment as part of its investigation as online marketplaces use users' data to gain market power.

The German order "serves the objective of protecting free competition" and was not about data protection, its lawyer Philipp Krueger said.

The CJEU court adviser will give her non-binding opinion on Sept. 20, with judges expected to follow with a ruling in the coming months.

The case is C-252/21 Meta Platforms and others.