A former Russian spy living in Britain has been hospitalised after being exposed to a mystery substance — sparking suspicions over what or who is to blame.

The prime theory is that it was an assassination attempt carried out on the orders of the Russian government. If true, it would not be the first time Russia has conducted assassinations on British soil.

Most famously, former Russian spy Alexander Litvinenko died in 2006 after being poisoned with polonium. A public inquiry subsequently concluding that Putin personally approved his murder. And a BuzzFeed News investigation has previously identified 14 deaths in the UK that American intelligence agencies have linked to Russia.

Speaking in the House of Commons on Tuesday, Foreign Secretary Boris Johnson said that while it was too early to say what caused Sergei Skripal and his companion's illness, it had "echoes" of Litvinenko's case and that "no attempt to take life on British soil will go unsanctioned or unpunished."

Here are the other times when Russia has been linked to deaths in Britain.

In the US, meanwhile, there was Mikhail Lesin, 2015. Lesin, a former adviser to Putin and founder of propaganda network Russia Today, was found dead in Washington DC in 2012. US authorities have said his death was the result of a drunken fall, but some FBI agents reportedly believe he was beaten to death.