In a joint statement, Britain, France, Germany, Sweden and the Netherlands said tests on samples taken from Navalny’s body had “conclusively” detected epibatidine, a toxin found in poison dart frogs in South America and not naturally present in Russia.
Russia, which has repeatedly denied responsibility for Navalny’s death, rejected the allegation.
The state news agency TASS cited officials describing it as “a Western propaganda hoax”, while the Russian embassy in London mocked the claim.
Britain said the suspected poisoning reflected “an alarming pattern of behaviour”.
The UK previously held a public inquiry into the 2018 poisoning in Britain of former Russian double agent Sergei Skripal, and last year concluded Russian President Vladimir Putin must have ordered the Novichok nerve agent attack.
TASS quoted Russian Foreign Ministry spokesperson Maria Zakharova as saying Moscow would comment once test results were available and the substance formulas disclosed, adding that, until then, such claims were propaganda intended to distract from Western problems.