7 crazy stories of Russia's enemies mysteriously getting poisoned - like when a Cold War dissident was hit with a poison-tipped umbrella

Britain has warned of a robust response if it finds evidence of Russian involvement in the collapse of a former Russian agent convicted of betraying dozens of spies to British intelligence.

Sergei Skripal and his daughter, Yulia, are critically ill after being exposed to what police called an unknown substance. Russia's embassy in London has expressed concern about British media reporting of the incident, which has included suggestions that Skripal and his daughter were poisoned.

Interestingly, if Russia is behind some or all of these poisonings, it is technically legal under Russian law. In 2006, the Russian Parliament passed a law allowing the Russian President to eliminate enemies of the state overseas, according to BuzzFeed News and BBC.

Below is a list of some previous incidents in which critics or enemies of Moscow have been victims of poisoning or suspected poisoning, or have cried foul after suddenly falling ill: