Russian opposition leader Alexei Navalny was poisoned by the Kremlin with a rare and lethal toxin found in the skin of poison dart frogs, five European countries said Saturday.

The foreign ministries of the U.K., France, Germany, Sweden and the Netherlands said analysis of samples taken from Navalny's body "conclusively confirmed the presence of epibatidine." It is a neurotoxin found in the skin of dart frogs in South America that is not found naturally in Russia, they said.

A joint statement said: "Russia had the means, motive and opportunity to administer this poison."

They five countries said they were reporting Russia to the Organization for the Prohibition of Chemical Weapons for a breach of the Chemical Weapons Convention.

The announcement came as Navalny's widow, Yulia Navalnaya, attended the Munich Security Conference in Germany as the second anniversary of Navalny's death approaches.

Navalny, who crusaded against official corruption and staged massive anti-Kremlin protests as President Vladimir Putin's fiercest foe, died in an Arctic penal colony on Feb. 16, 2024, while serving a 19-year sentence that he believed to be politically motivated.

"Russia saw Navalny as a threat," British Foreign Secretary Yvette Cooper said. "By using this form of poison the Russian state demonstrated the despicable tools it has at its disposal and the overwhelming fear it has of political opposition."

The poisoning of Navalny shows "that Vladimir Putin is prepared to use biological weapons against his own people in order to remain in power," French Foreign Minister Jean-Noël Barrot wrote on X.

Navalny's widow said last year that two independent labs had found that her husband was poisoned shortly before he died. She has repeatedly blamed Putin for her husband's death. Russian officials have vehemently denied the accusation.

Yulia Navalnaya said Saturday that she had been "certain from the first day" that her husband had been poisoned, "but now there is proof."

"Putin killed Alexei with chemical weapon," she wrote on social network X. She said Putin was "a murderer" who "must be held accountable."

Russian authorities said that the politician became ill after a walk and died from natural causes.

Epibatidine is found naturally in dart frogs in the wild, and can also be manufactured in a lab, which European scientists suspect was the case with the substance used on Navalny. It works on the body in a similar way to nerve agents, causing shortness of breath, convulsions, seizures, a slowed heart rate and ultimately death.

Navalny was the target of an earlier poisoning in 2020, with a nerve agent in an attack he blamed on the Kremlin, which always denied involvement. His family and allies fought to have him flown to Germany for treatment and recovery. Five months later, he returned to Russia, where he was immediately arrested and imprisoned for the last three years of his life.

The U.K. has accused Russia of repeatedly flouting international bans on chemical and biological weapons. It accuses the Kremlin of carrying out a 2018 attack in the English city of Salisbury that targeted a former Russian intelligence officer, Sergei Skripal, with the nerve agent Novichok. Skripal and his daughter became seriously ill, and a British woman, Dawn Sturgess, died after she came across a discarded bottle with traces of the nerve agent.

A British inquiry concluded that the attack "must have been authorized at the highest level, by President Putin."

The Kremlin has denied involvement. Russia also denied poisoning Alexander Litvinenko, a former Russian agent turned Kremlin critic who died in London in 2006 after ingesting the radioactive isotope polonium-210. A British inquiry concluded that two Russian agents killed Litvinenko, and Putin had "probably approved" the operation.