BERLIN — The Russian opposition leader Aleksei A. Navalny, currently under treatment in a German hospital, was poisoned with a deadly nerve agent from the Novichok family, the German government said on Wednesday.
Citing what it called “unequivocal evidence,” Berlin demanded an explanation from Moscow in a case that seems bound to raise tensions once more between Russia and the West.
“Mr. Navalny has been the victim of a crime,” Chancellor Angela Merkel of Germany said in a statement. “It raises very serious questions that only the Russian government can and must answer.”
Novichok, a Soviet-era weapon invented for military use, was used against Sergei V. Skripal, a former Soviet spy, and his daughter in a 2018 attack in Salisbury, England, that the British government attributed to Russia’s military intelligence arm, the G.R.U.
Leonid Volkov, Mr. Navalny’s chief of staff, echoed that view in a Twitter post on Wednesday, saying, “In 2020, poisoning Navalny with Novichok is the same as leaving an autograph at the scene of the crime.”
Toxicology tests carried out by a German Army laboratory revealed the “doubtless presence of a nerve agent from the Novichok group” in the system of Mr. Navalny, who was flown to Germany on Aug. 22 after he collapsed on a flight from Siberia to Moscow.
“The German government condemns this attack in the strongest possible terms,” Steffen Seibert, a spokesman for Chancellor Angela Merkel, said in a statement. “The Russian government is urgently requested to explain what happened.”
The Kremlin said it had not been not informed of Germany’s findings before they were announced, the Russian state news outlet Tass reported. “No, such information was not conveyed to us,” the presidential spokesman, Dmitri S. Peskov, said.
Heiko Maas, the German foreign minister, said he would summon the Russian ambassador to inform him of the lab results.
Also informed of the findings were Mr. Navalny’s wife, Yulia, who flew with him to Berlin, and the doctors who are treating him at the Charité hospital, where he remains in critical but stable condition in a medically induced coma.
His team of doctors said in a statement on Wednesday that they expected a lengthy recovery and that they could not rule out lasting effects.
The United States stopped producing nerve agents in 1970, after the development of “third generation” nerve agents like sarin and VX, but Soviet scientists kept at it for two decades, developing a “fourth generation,” the Novichok group or family of toxins.
Developed for battlefield use against Western troops, Novichok has come to be associated with state-sponsored poisonings of those who fall out of favor with the Kremlin. Exposure to Novichok agents leads to muscle spasms, secretion of fluid into the lungs and organ failure, chemical weapons experts say.
The case is expected to further strain ties between Berlin and Moscow that have been tense since Russia annexed Crimea in 2014. Ms. Merkel said on Wednesday that Germany, which currently holds the European Union’s rotating presidency, would consult with its European and NATO partners about a coordinated response. “The world will wait for answers,” she said.
In March 2018, Mr. Skripal and his daughter, Yulia Skripal, were found unconscious and twitching on a park bench in Salisbury and were later found to have Novichok in their systems. Both survived the attack.
There had been some question whether his poisoning had been carried out by Russian agents or perhaps by freelancers hoping to curry favor with President Vladimir V. Putin. The confirmed presence of a closely guarded nerve agent appeared to settle that dispute, pointing directly to a state actor.
Andrew E. Kramer contributed reporting from Moscow, Megan Specia from London and Christopher F. Schuetze from Berlin.