Alexei Navalny

On the second anniversary of his death, Lyudmila Navalnaya says her son did not "simply die in prison".

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

The foreign secretary said the UK would "continue to look at co-ordinated action, including increasing sanctions on the Russian regime".

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

Yulia Navalnaya is urging the two laboratories she says found her husband was poisoned to release their findings.

For years, Navalny was Vladimir Putin's biggest political rival, but now the Russian opposition is weaker than ever.

The trio were arrested as authorities intensified pressure on the jailed Kremlin critic, who died last February.