"Only the Russian state had the means, motive and opportunity," the U.K. said. Navalny’s widow said, "proof: Putin killed Alexei."

Five European countries say tests confirm Alexei Navalny was poisoned with a lethal toxin, and they blame the Russian state

There is no innocent explanation for the toxin, called epibatidine, being found in samples taken from Navalny's body, the Foreign Office said.

The Russian state killed opposition leader Alexei Navalny using a poison developed from a dart frog toxin, the Foreign Office has said.

"Only the Russian government had the means, motive and opportunity to deploy this lethal toxin against Alexei Navalny during his imprisonment in Russia," she said.