The German state of Schleswig-Holstein is the latest party to throw a spanner in the works of Google's proposed buyout of online advertising firm DoubleClick.

Thilo Weichert, Schleswig-Holstein's data protection commissioner, claimed that the $3.1bn (£1.52bn) merger will threaten user privacy.

Weichert insisted in an open letter to European Union Competition Commissioner Neelie Kroes that the merging of the two companies' databases would represent a real threat.

"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.

The proposed merger has been criticised by the Electronic Privacy Information Center which filed an official complaint to the Federal Trade Commission in April.

Advertising rival Microsoft called for an antitrust investigation into the deal in April 2007, and the FTC announced a month later that it would investigate the merger.