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.
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.
At the time of the Skripal poisoning, experts said that the stockpile of Novichok was tightly guarded, and expressed doubts that the substance would be used by anyone other than a state-sponsored agent.
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 has said that Germany, which currently holds the European Union’s rotating presidency, will try to seek a joint European response to Mr. Navalny’s case.
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.
Investigators believed that the Skripals fell ill after a liquid form of the nerve agent that was applied to the front door knob of Mr. Skripal’s home seeped through their skin.
Four months later, two British citizens were poisoned after they retrieved a discarded vial of the chemical from a trash can, and one of them later died. Two police officers also became ill as a result of exposure to the chemical.
The Skripal case set off a major diplomatic crisis between London and Moscow, with Britain and its allies expelling dozens of Russian diplomats and imposing punishing sanctions on Russia.
Mr. Navalny, the most persistent critic of President Vladimir V. Putin of Russia, fell ill on Aug. 20 after spending several days meeting with opposition candidates in Novosibirsk, Siberia’s largest city. He had been promoting a strategy aimed at drawing support from the dominant United Russia party at the local level before nationwide municipal elections on Sept. 13.
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.