BERLIN — Russian opposition leader Alexei Navalny was poisoned with the same type of Soviet-era nerve agent that British authorities identified in a 2018 attack on a former Russian spy, the German government said Wednesday, citing new test results.

The Berlin hospital treating Navalny said he remains in a serious condition though the symptoms of his poisoning are declining. It said that it expects a long recovery, and it still can’t rule out long-term consequences from the poisoning.

Chancellor Angela Merkel’s spokesman, Steffen Seibert, said that testing by a special German military laboratory at the request of the Berlin hospital where Navalny remains in a coma showed “proof without doubt of a chemical nerve agent from the Novichok group.”

“It is a dismaying event that Alexei Navalny was the victim of an attack with a chemical nerve agent in Russia,” Seibert said in a statement. “The German government condemns this attack in the strongest terms.”

Navalny, a politician and corruption investigator who is one of Russian President Vladimir Putin’s fiercest critics, fell ill on a flight back to Moscow from Siberia on Aug 20 and was taken to a hospital in the Siberian city of Omsk after the plane made an emergency landing.

He was transferred two days later to Berlin’s Charité hospital, where doctors last week said initial tests indicated Navalny had been poisoned.

British authorities identified Novichok as the poison used in 2018 on former Russian spy Sergei Skripal and his daughter in England. The nerve agent is a cholinesterase inhibitor, part of the class of substances that doctors at Charite said last week had shown up in Navalny’s system.

The Kremlin Remained Tight-Lipped and Said It Hadn’t Been Informed

German Foreign Minister Heiko Maas said the Russian ambassador was summoned to his ministry Wednesday and told in “unmistakable” terms of Germany’s call for the Navalny case to be investigated “in full and with full transparency.”

“We now know that there was an attack with a chemical nerve agent,” Mass said. “That makes it even more urgent to determine who was responsible in Russia and to hold them to account.”

The Kremlin remained tight-lipped and said it hadn’t been informed, even though its ambassador in Berlin had been summoned.

Seibert said the German government would inform its partners in the European Union and NATO about the test results. He said it wants to consult with them in light of the Russian response “on an appropriate joint response.” Germany also will contact the Organization for the Prohibition of Chemical Weapons, he added.

“Russia itself should have a serious interest in good relations with its neighbors in Europe,” said Maas, the foreign minister. “Now at the very latest, it is the time to make a decisive contribution toward this.”

In an update shortly after the government’s announcement, the Charite hospital said that Navalny is still in intensive care and remains on a ventilator. But it said that he “continues to improve.”

“Recovery is likely to be lengthy,” it said in a statement. “It is still too early to gauge the long-term effects, which may arise in relation to this severe poisoning.”

Navalny’s allies in Russia have insisted he was deliberately poisoned by the country’s authorities, accusations that the Kremlin rejected as “empty noise.”

“To poison Navalny with Novichok in 2020 would be exactly the same as leaving an autograph at a crime scene, like this one,” Navalny’s longtime ally and strategist Leonid Volkov said in a tweet that featured a photo of Putin’s name and a signature next to it.

Novichok Is a Class of Military-Grade Nerve Agents Developed by the Soviet Union at the End of the Cold War

The Russian doctors who treated Navalny in Siberia repeatedly contested the German hospital’s poisoning conclusion, saying they had ruled out poisoning as a diagnosis and that their tests for cholinesterase inhibitors came back negative.

Novichok is a class of military-grade nerve agents developed by the Soviet Union at the end of the Cold War. Western weapons experts say it was only ever manufactured in Russia. After the Skripals were poisoned, Russia said the U.S., Britain and other Western countries acquired the expertise to make the nerve agent after the Soviet Union collapsed, and that the Novichok used in the attack could have come from them.

According to the Organization for the Prohibition of Chemical Weapons, there is no record of Novichok having been declared by any nation that signed the chemical weapons convention.

“Unless you are working for the military, it is impossible to be accidentally exposed,” Richard Parsons, a senior lecturer in biochemical toxicology at King’s College London, said. “It is only dangerous when it is about to be used, i.e. mixed together. It is unavailable from anywhere except the Russian military as far as I am aware.”

Britain charged two Russians — alleged to be agents of the Russian military intelligence service GRU — in absentia with the 2018 attack that left the Skripals in critical condition and killed a local woman. Russia has refused to extradite the men to the U.K.

British police believe the nerve agent was smuggled to Britain in a counterfeit Nina Ricci perfume bottle and sprayed on the front door of Sergei Skripal’s house in the city of Salisbury in southwest England.

More than three months later, the bottle was found by a local man, 48-year-old Charlie Rowley. He was hospitalized and his girlfriend, Dawn Sturgess, 44, died after being exposed to the contents.