Five European countries — United Kingdom, France, Germany, Sweden and Netherlands — said tests showed that Aleksei A. Navalny was poisoned with epibatidine, a toxin found in South American poison dart frogs, directly challenging Russia’s claim that he died of natural causes in an Arctic prison in 2024.

The governments stated that only Russia had the means, motive and opportunity to deploy the toxin and accused it of continuing to use chemical weapons.

Russian officials dismissed the allegations as a political campaign and said they would not respond without detailed test results.

Navalny had previously survived a 2020 poisoning with Novichok and had remained President Vladimir V. Putin’s most prominent political opponent, The New York Times has reported.

His widow, Yulia Navalnaya, called the findings proof that Putin was personally responsible for her husband’s death.