Navalny is being treated at a German hospital after falling ill.

Alexey Navalny, the Russian opposition leader and one of Vladimir Putin’s fiercest critics, was poisoned with a chemical nerve agent from the Novichok group, the German government said, the same type of Russian military nerve agent weapon used in the poisoning of a former Russian double agent, Sergey Skripal, in Britain in 2018.

Navalny, 44, is being treated at the Berlin university hospital where he was transported to after falling ill on a plane in Siberia two weeks ago. He remains in an induced coma in the hospital’s intensive care unit.

The Russian doctors initially treating Navalny in a Siberian hospital said there was no evidence he had been poisoned.

However, a spokesman for Germany’s federal government, Steffen Seibert, in a statement released Wednesday said that tests by a German military lab had provided “unequivocal proof” that Navalny had been poisoned with “a chemical nerve warfare agent of the Novichok group.”

Novichok is the name of a number of nerve agents developed as part of a secret Soviet chemical weapons program. A Novichok nerve agent was used to poison the ex-Russian spy Sergey Skripal and his daughter in March 2018 in the British town of Salisbury, an attack that British authorities have said was carried out by agents from Russia’s military intelligence agency.

Germany said it will inform NATO and European Union countries about the result of its investigation and would discuss “an appropriate joint reaction.”

“It is a shocking event that Alexei Navalny was the victim of an attack with a chemical nerve agent in Russia. The federal government condemns this attack in the strongest possible terms,” the German government statement said, saying it would summon Russia’s ambassador for an explanation. “The Russian government is urged to explain itself about the incident.

It said Germany’s government would also contact the Organization for the Prohibition of Chemical Weapons (OVCW).

Russia’s government has denied any involvement in Navalny’s sickness and has already cast doubt on the idea that he might have been poisoned.

Following Skripal’s poisoning in 2018, NATO countries expelled dozens of Russian diplomats as part of a coordinated response. Russia has always denied it was behind Skripal’s poisoning, despite extensive evidence linking Russian military intelligence agents who visited Skripal’s house in Salisbury shortly before his poisoning.

Navalny’s colleagues and relatives have earlier accused the Kremlin of being directly involved in Navalny’s poisoning and accused Russian authorities of a cover-up. Doctors in the Siberian hospital Omsk where Navalny was being treated have repeatedly said they found no evidence of poisoning and had suggested instead he had suffered a sudden fall in blood sugar.

Shortly after Navalny arrived in Berlin’s Charité hospital for treatment, however, doctors there said they believed he had been poisoned with cholinesterase inhibitors, a broad class of chemicals that includes nerve agents, including Novichok.

Navalny is Russia’s most prominent opposition leader, who made his name through investigations of alleged corruption by powerful Russians, including top officials and some of Putin’s inner circle. He also built a grassroots opposition movement that has helped lead some of the largest anti-Putin protests in Moscow in recent years.