BERLIN (Reuters) -German Economy Minister Robert Habeck's constituency office said on Thursday it had received an envelope containing a white substance, prompting a large-scale police operation.

After a preliminary examination police believe the substance was merely finely-ground sand, a spokesperson said, pending a full investigation of the incident.

Employees of Habeck's office in Flensburg, near the northern border with Denmark, brought the envelope on Wednesday afternoon to police who subsequently alerted the fire brigade, a police statement said late that day.

The police operation lasted two hours and the substance was removed from the building by the fire brigade in a container.

Habeck, whose ministry is responsible for energy as well as the economy, and who also serves as deputy chancellor of Germany, has been the face of high-profile policies seeking to mitigate the economic fallout of Russia's invasion of Ukraine.

His ranking in opinion polls has fallen in recent weeks following a nepotism scandal in his ministry and criticism of his push to phase out older home-heating systems.

(Reporting by Rachel More, writing by Friederike Heine; editing by Linda Pasquini, Alex Richardson and Mark Heinrich)