A conservative member of the German parliament and prominent critic of Russian President Vladimir Putin died of heart failure or overheating after falling asleep in a sauna and there are no signs of foul play, state prosecutors said today.

Andreas Schockenhoff, a deputy parliamentary floor leader for Chancellor Angela Merkel's conservative party and former point-man for the German government on relations with Russia, died on Saturday at age 57. He was an outspoken proponent of tough sanctions against Russia after its annexation of Crimea.

A court in his home town of Ravensburg ordered the autopsy to rule out any link between his death and his hardline stance towards the Kremlin.

"There are no indications of any third party involvement or violence," said Ravensburg state prosecutor Karl-Josef Diehl in a statement based on preliminary findings. "A toxicological report is still pending and will be available in a few days."

The state prosecutors said that based on the preliminary findings Schockenhoff died of heart failure or massive overheating in a sauna, possibly after falling asleep.

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 Putin, the Russian Foreign Ministry refused to recognise him as a representative of the German government.

In a German radio interview 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."