The toxin, epibatidine, can be found in frogs in the wild in South America or manufactured in a lab.

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

Navalny, who staged massive anti-Kremlin protests as President Vladimir Putin's fiercest foe, died in the Arctic penal colony in February 2024.