Thilo Weichert, Schleswig-Holstein's data protection commissioner, claimed that the $3.1bn (£1.52bn) merger will threaten user privacy.
"At present we have to assume that, in the event of a takeover of DoubleClick, its databases will be integrated into Google's, with the result that fundamental provisions of the European Data Protection Directive will be violated," he said.
Weichert maintained that the link-up contravened the EU's fundamental data privacy principles, such as limited specific use, transparency, the right to object, the protection of sensitive data and the right to have data deleted.
Cotton seedling freezes to death as Chang'e-4 shuts down for the Moon's 14-day lunar night
Fortnite easily out-earns PUBG, Assassin's Creed Odyssey and Red Dead Redemption 2 in 2018
Meteor showers as a service will be visible for about 100 kilometres in all directions
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago