Antitrust authorities may assess whether companies comply with EU data protection rules during their investigations, an adviser to Europe's top court has ruled, dealing a possible blow to Facebook owner Meta in its fight against the German competition agency.

The non-binding opinion, if followed by the Court of the Justice of the Europe Union (CJEU) in the coming weeks, could further embolden the German enforcer and its peers in other EU countries to widen their scrutiny of Big Tech based on privacy rules.

Meta's battle with the German cartel office started in 2019 after the latter 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 company, which owns Facebook, Instagram and WhatsApp, challenged the finding. At issue is whether the German antitrust agency overstepped its authority by using its antitrust power to address data protection concerns, which are the remit of national data protection authorities.

A German court subsequently sought guidance from the CJEU.

Advocate General Athanasios Rantos at the Court of Justice of the European Union (CJEU) said whether a company complies with the bloc's landmark privacy rules may be an important indication of whether its conduct breaches competition rules.

"A competition authority may, in exercising its powers, take account of the compatibility of a commercial practice with the General Data Protection Regulation," he said in a non-binding opinion.

He said antitrust watchdogs however need to consult with the lead privacy enforcers as well, which in Meta's case is the Irish data protection agency as its European headquarters is based there.

"However, it must take into account any decision or investigation by the competent authority under that Regulation," he said.

Meta said: "We await the final judgment to determine any next steps."

The court, which usually follows the majority of such recommendations, will rule in the coming months.

Meta is also in the German watchdog's crosshairs for linking its Oculus virtual reality products with the social network and its platform.

In May, it was designated by the German authority as of "paramount importance for competition across markets" subject to tougher rules.

The case is C252/21 Meta Platforms and others (general conditions of use of a social network).