A senior member of German Chancellor Angela Merkel's government resigned from his political posts this weekend after police said he was under investigation for possessing and distributing child pornography on his mobile phone.

Jörg Tauss, a Social Democrat in the grand coalition, quit as his party's parliamentary media affairs and education spokesman and as the regional party general secretary for Baden-Württemberg state after police raided his Berlin and Karlsruhe offices.

Tauss was stripped of his parliamentary immunity on Thursday when news of the raid broke. He said he had resigned, six months before Germany's general election in September, "to avert damage to my party and my parliamentary group". He has campaigned against online child pornography and said he was "absolutely sure" he could be quickly cleared of the accusations.

Prosecutors rejected Tauss's claims that the investigation was a result of his campaign against child pornography. A prosecutor with knowledge of the case said officers had found incriminating material in Tauss's Berlin apartment which demanded an explanation. "It clearly has no connection with his activities as an MP."

Reports said the investigation had been launched after prosecutors in the northern city of Bremerhaven found Tauss's phone numbers and his Berlin address in the possession of a man already suspected of involvement in child pornography distribution.

It is believed to be the first investigation of a German MP over such an emotive, potentially explosive political issue.

Investigators are said to have established that Tauss and the other man contacted each other 23 times on mobile phones. The MP was said to have sent the man a child pornography video clip and received a DVD in return.

Just before his summer recess, Tauss was said to have sent the man a text message and then phone him two days after his holiday ended. Prosecutors refused to confirm the reports.

Tauss pledged to co-operate with investigators: "I have always fought hard for better protection of children and youths on the internet and for a decisive campaign against child pornography," he said on his website.