Russian opposition activist Alexei Navalny was poisoned with a substance that can be used as a nerve agent before falling into a coma last week, German doctors treating him said Monday.
The fierce critic of Russian President Vladimir Putin collapsed on a plane in Russia last Thursday.
His supporters accuse the Kremlin of being behind his poisoning. Russian officials deny that, and the Russian doctors who initially treated him said they found no signs of poisoning during tests last week.
Following Monday's announcement, German Chancellor Angela Merkel called on Russian authorities to investigate the incident and bring those responsible to justice.
She said in a statement that this was particularly urgent "in view of Mr. Navalny's prominent role in the political opposition in Russia." Earlier her spokesman, Steffen Seibert, said that "recent Russian history has had several such suspected cases."
Doctors at Berlin's Charité hospital said multiple tests showed Navalny, 44, had been poisoned with a type of chemical called a cholinesterase inhibitor. The exact substance is not yet clear and they said they are performing further tests.
Although there is "currently no acute danger to his life" the doctors could not rule out long-term damage to his nervous system, the statement said.
Cholinesterase inhibitors are chemicals that slow or stop the breakdown of the main neurotransmitter in the nervous system.
They are found in pesticides and drugs that treat conditions such as dementia. But they are also in nerve agents like sarin and Novichok, which was used to poison the Russian double agent Sergei Skripal in the United Kingdom in 2018, an incident the British government blamed on the Kremlin.
The doctors in Germany have treated Navalny with the antidote atropine, the same remedy used to aid Skripal's recovery.
Navalny became seriously ill on a flight from Siberia to Moscow that was forced to make an emergency landing in the city of Omsk. His associates said he was poisoned after drinking black tea at an airport cafe, and demanded he be moved to Germany where he could be treated safely.
The Russian doctors said he was too unstable to be moved and said his condition may have been caused by low blood sugar, not poison. Supporters saw this as stalling and buying time for any substance to leave his system.
On Friday, his wife, Yulia, appealed to the Kremlin directly, and on Saturday officials consented to him being airlifted to Berlin where he is currently being treated.
Navalny has been among the Kremlin's most vocal critics of the past decade, with his anti-corruption foundation conducting in-depth investigations into the highest ranks of the Russian political elite. It is not the first time his supporters have alleged foul play.
Last year, when Navalny was serving a prison sentence, he was rushed to hospital with what his team said was a suspected poisoning. Doctors then said he had a severe allergic attack. In 2017, he was attacked by several men who threw antiseptic in his face, damaging one eye.
The Associated Press contributed to this report.