Germany has produced “unequivocal evidence” that Russian opposition leader Alexei Navalny was poisoned with the banned nerve agent Novichok.
This has led German Chancellor Angela Merkel to conclude Mr Navalny was the victim of “attempted murder by poisoning”.
A government spokesman had earlier confirmed he was attacked with the same Soviet-era chemical weapon used two years ago to poison Sergei Skripal, a Russian spy living in Britain, and his daughter.
A critic of President Vladimir Putin, Mr Navalny has been lying in a coma in a Berlin hospital for about two weeks, with his supporters adamant something was put in his tea at an airport cafe.
The Germans carried out extensive tests at a military laboratory which confirmed Mr Navalny was poisoned with Novichok – a deadly group of nerve agents developed by the Soviet military in the 1970s and 1980s.
Given the 1997 Chemical Weapons Convention, of which Moscow is a signatory, bans the weaponisation of any chemical, Germany’s government will inform the EU and NATO of the “unequivocal evidence” that Mr Navalny was attacked with Novichok.
The chemical “causes a slowing of the heart and restriction of the airways, leading to death by asphyxiation”, according to Professor Gary Stephens, a pharmacology expert at the University of Reading.
Russian prosecutors last week said there was no need for a criminal investigation as they had found no sign that any crime had been committed.
Germany “will discuss an appropriate joint response with the partners in the light of the Russian response”, government spokesperson Steffen Seibert said in a statement.
The Russian rouble extended losses against the euro after the German government statement.
German Foreign Minister Heiko Maas urged Russia to investigate Navalny’s poisoning now that clinical tests had shown he had been attacked with a chemical nerve agent.
“This makes it all the more urgent that those responsible in Russia be identified and held accountable,” Mr Maas told reporters.
“We condemn this attack in the strongest terms.”
A Kremlin spokesman said it had not been informed that Germany believed Navalny had been poisoned with Novichok, according to the RIA news agency.
Russian President Vladimir Putin’s spokesman said on Wednesday that Russian authorities were ready to fully co-operate with Germany.
“On the whole, we confirm that we are ready and interested in fully co-operating and exchanging information on the matter with Germany,” spokesman Dmitry Peskov said in comments carried by state news agency TASS.
Russia has previously declined to conduct a criminal investigation into the allegation that Navalny was poisoned, as Russian officials say that no hard evidence has been found.
Mr Peskov reiterated that stance in the comments to state media: “Before the patient was transferred to Berlin, a wide range of tests had been done in our country in conformity with all international norms but no poisonous substances were found.”
Novichok was developed in the Soviet Union in the 1970s and 1980s and is believed to be five to 10 times more lethal than gases VX and sarin.
There are no known uses before 2018 when Novichok was deployed in Britain on a former Russian spy and his daughter.
The chemical agents was made with agrochemicals so that its production could more readily be hidden within a legitimate commercial industry, according to US chemical weapons expert Amy Smithson.
Russia was once believed to possess thousands of tonnes of weaponised Novichok varieties and their precursors, according to a 2014 report by the US-based Nuclear Threat Initiative, a non-partisan group working to reduce the threat of weapons of mass destruction.