Former Russian spy Sergei Skripal was critically ill on Monday (March 5) after exposure to an unidentified substance in Britain.

Mr Skripal, 66, was once a colonel in Russia's GRU military intelligence service. He was convicted in Russia of betraying agents to British intelligence in 2006.

He was sentenced to 13 years in prison, but he was later pardoned in 2010 as part of a Cold War-style spy swap on the tarmac of Vienna airport.

Here are five other high-profile poison and death cases involving Kremlin opponents:

1. Alexander Litvinenko

Mr Alexander Litvinenko, a former Russian spy who fled to Britain, died in 2006 after being exposed to polonium, a rare and highly radioactive metal.

The 43-year-old had drunk green tea laced with the poison at London's Millennium Hotel, where he had been living in exile.

He was known for previously criticising Russian President Vladimir Putin of being corrupt.

A British inquiry in January 2016 found that Mr Putin had "probably approved" of the killing of Mr Litvinenko.

2. Roman Tsepov

Russian businessman Roman Tsepov, 42, died after drinking a cup of tea at a Federal Security Service (Russia's successor agency to the KGB) office in Saint Petersburg on Sept 11, 2004.

Mr Tsepov was a one-time associate of Mr Putin and had previously been arrested for illegal storage of weapons and drugs.

A post mortem found that he had been poisoned by an unnamed radioactive material.

3. Boris Nemtsov

Russian opposition leader Boris Nemtsov, one of Mr Putin's most vocal critics, was shot dead on Feb 27, 2015 as he walked across a bridge near the Kremlin.

Aged 55, he had been working on a report examining Russia's role in the conflict in Ukraine.

Five Chechen men were convicted of the killing in 2017, but Mr Nemtsov's family believes his real killer remains at large, and that he was assassinated for his political work.

4. Anna Politkovskaya

Russian investigative journalist Anna Politkovskaya was shot in her Moscow apartment building in 2006.

Ms Politkovskaya had chronicled corruption under Mr Putin and human rights abuses during Russia's conflict with separatists in Chechnya.

There were suspicions that Mr Putin was involved, but five men - including four members of the same Chechen family - were eventually found guilty of the murder.

However, Ms Politkovskaya's family and former colleagues remain convinced the masterminds have not been brought to justice.

5. Stanislav Markelov and Anastasia Baburova

Human rights lawyer Stanislav Markelov was gunned down in the middle of a Moscow street after leaving a press conference in 2009. Opposition journalist Anastasia Baburova, 25, who was with him, was also shot dead.

Mr Markelov, 34, who had represented victims of a 2002 Moscow theatre siege, where over 100 hostages were killed by Russian special forces, was assumed to be the main target. He also worked for opposition newspaper Novaya Gazeta and had previously dealt with the murder case of the paper's correspondent Politkovskaya.