A German court has ordered an autopsy on a conservative member of parliament who was a prominent critic of Russia's Vladimir Putin in order to rule out any link between his death and his hardline stance towards the Kremlin, state prosecutors said.

Andreas Schockenhoff, a deputy parliamentary floor leader for Chancellor Angela Merkel's conservative party, died unexpectedly on Saturday at the age of 57.

A former point-man for the German government on relations with Russia, Schockenhoff was an outspoken proponent of tough sanctions against Russia after its annexation of Crimea.

"We don't know the cause of his death," Karl-Josef Diehl from the state prosecutor's office in Ravensburg told Reuters.

"In order to determine the exact cause of death and to counter all doubts and speculation, including any related to his previous work, like his role as the government's coordinator for Russia, an autopsy was requested and approved by the court."

Diehl said the coroner would perform the autopsy on Tuesday and results would be announced on Wednesday. The cause of death was unknown, he said. German media reported he had died of natural causes.

Schockenhoff became a national figure in 2011 when he publicly acknowledged that he had an alcohol problem after crashing his car while driving under the influence. He left politics for a month for treatment.

Schockenhoff was the German government's coordinator for Russia from 2006 until early 2014. Two years ago, after he authored a parliamentary report that criticised president Putin, the Russian Foreign Ministry refused to recognise him as a representative of the German government.

In an interview with Deutschlandfunk radio last month, Schockenhoff said Putin was waging "hybrid warfare" against Ukraine and the West.

"Russia will only respond when pressure is applied," he said. "And Putin will only do what he's forced to do."

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