Russian opposition politician Alexei Navalny was poisoned with a Novichok nerve agent, Germany’s government says.
It said toxicology tests at a military laboratory showed “unequivocal proof” of an agent from the Novichok group.
Mr Navalny was airlifted to Berlin for treatment after falling ill during a flight in Russia’s Siberia region last month. He has been in a coma since.
His team says he was poisoned on President Vladimir Putin’s orders. The Kremlin has dismissed the allegation.
The German government said it condemned the attack in the strongest terms and called for Russia to urgently provide an explanation.
“It is a disturbing development that Alexei Navalny was the victim of a chemical nerve agent in Russia,” it said.
Chancellor Angela Merkel has met senior ministers to discuss the next steps, the statement said.
The Kremlin said it had not received any information from Germany that Mr Navalny had been poisoned using a Novichok nerve agent, Russia’s Tass news agency reported.
The German government said it would inform the EU and Nato military alliance of its findings.
“[The federal government] will discuss an appropriate joint response with the partners in the light of the Russian response,” it said.
Mr Navalny’s wife Yulia Navalnaya and Russia’s ambassador to Germany would also be informed of the findings, the statement said.
What happened to Navalny?
Mr Navalny fell ill on a flight from Tomsk to Moscow. His supporters suspect poison was placed in a cup of tea at Tomsk airport.
The flight of the prominent Putin critic was diverted to Omsk, where doctors treated him for three days before he was transferred to the Charité hospital in Berlin.
The Kremlin says Russian doctors administered atropine – which can be used to treat the effects of nerve agents – but found no evidence of poisoning.
Mrs Navalnaya said she feared Russian doctors had delayed his transfer as authorities were trying to wait for evidence of any chemical substance to disappear.
Doctors at Charité hospital have said his condition is serious but not life-threatening.
What is Novichok?
The name Novichok means “newcomer” in Russian, and applies to a group of advanced nerve agents developed by the Soviet Union in the 1970s and 1980s.
Novichok agents have similar effects to other nerve agents – they act by blocking messages from the nerves to the muscles, causing a collapse of many bodily functions.
While some Novichok agents are liquids, others are thought to exist in solid form. This means they can be dispersed as an ultra-fine powder.
Novichoks were designed to be more toxic than other chemical weapons, so some versions begin to take effect rapidly – in the order of 30 seconds to two minutes.
In 2018 an ex-Russian spy, his daughter and a couple were poisoned with a Novichok agent in the British city of Salisbury. Russian suspects identified in the case are alleged to have smeared the nerve agent on the door handle of Sergei Skripal’s home.
The attack left Mr Skripal and his daughter Yulia critically ill. Dawn Sturgess, 44, was later exposed to the same nerve agent and died in hospital.
Who is Navalny?
Mr Navalny is an anti-corruption campaigner who has led nationwide protests against the Russian authorities. He has called Mr Putin’s party a place of “crooks and thieves” that is “sucking the blood out of Russia”.
However, he has been banned from standing against Mr Putin in elections because of a conviction for embezzlement. He denies the crime, saying his legal troubles are Kremlin reprisals for his fierce criticism.
There have been a number of previous attacks on high-profile critics or opponents of President Putin, including politicians, intelligence officers and journalists. The Kremlin has always denied involvement.