Navalny fell ill on board a flight between Siberian Tomsk and Moscow on August 20.
He flew after a couple of days to the German capital Berlin, where he is now in a coma at the Charité Hospital.
German doctors have determined that Navalny was poisoned with the Soviet-developed neurotoxin Novichok.
It is the same poison that was used against the former Russian double agent Sergei Skripal and his daughter in the English city of Salisbury two years ago.
Due to the poisoning of Skripal, several Russian diplomats were expelled by NATO countries.