SAMPLES taken from Russian opposition leader Alexei Navalny showed the presence of the Soviet-era nerve agent Novichok, the German government has said.

Mr Navalny, a politician and corruption investigator who is one of Russian President Vladimir Putin's fiercest critics, fell ill on a flight back to Moscow from Siberia on August 20 and was taken to hospital in the Siberian city of Omsk after the plane made an emergency landing.

He was later transferred to Berlin's Charite hospital, where doctors last week said there were indications he had been poisoned.

Chancellor Angela Merkel's spokesman, Steffen Seibert, said in a statement that testing by a German military laboratory had shown proof of "a chemical nerve agent from the Novichok group".

Novichok was used to poison former Russian spy Sergei Skripal and his daughter in Britain. It is a cholinesterase inhibitor, part of the class of substances that doctors at the Charite initially identified in Mr Navalny.

Mr Seibert said the German government will inform its partners in the European Union and Nato about the test results, and will consult with its partners in light of the Russian response "on an appropriate joint response".

Mr Navalny's allies in Russia have insisted he was deliberately poisoned by the country's authorities, accusations the Kremlin rejected as "empty noise".

The Russian doctors who treated him in Siberia have repeatedly contested the German hospital's conclusion, saying they had ruled out poisoning.