LONDON (Reuters) - 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.

A still image taken from video shows Sergei Skripal, a former colonel of Russia's GRU military intelligence service, attending a hearing at the Moscow military district court, Russia August 9 2006. RTR/via Reuters TV

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.

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.

UNIDENTIFIED SUBSTANCE

Skripal, 66, was once a colonel in Russia’s GRU military intelligence service. He and his 33-year-old daughter were found unconscious on a bench outside a shopping center in the English city of Salisbury on Sunday. Police are investigating what made them ill.

The Kremlin says it is ready to cooperate if Britain asks it for help investigating the incident, and that it has no information about it. Asked about British media speculation that Russia had poisoned Skripal, a spokesman for President Vladimir Putin said: “It didn’t take them long.”

POISON-TIPPED UMBRELLA

During the Cold War, Georgi Markov, a Bulgarian dissident, was killed with a poison-tipped umbrella. Markov, a writer, journalist and opponent of Bulgaria’s then communist leadership, died on Sept. 11, 1978 after someone fired a ricin-laced pellet into his leg on London’s Waterloo Bridge.

According to accounts of the incident, Markov, who defected to the West in 1969, was waiting for a bus when he felt a sharp sting in his thigh. A stranger fumbled behind him with an umbrella he had dropped and mumbled “sorry” before walking away.

Markov later died of what is believed to be ricin poisoning, for which there is no antidote. Dissidents accused the Soviet KGB of being behind the killing.

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POLONIUM-210

Ex-KGB agent Alexander Litvinenko, 43, died after drinking green tea laced with polonium-210, a rare and potent radioactive isotope, at London’s Millennium Hotel.

Russian President Vladimir Putin probably approved the killing, a British inquiry concluded in 2016. The Kremlin has denied involvement.

An inquiry led by a senior British judge found that former KGB bodyguard Andrei Lugovoy and another Russian, Dmitry Kovtun, carried out the killing as part of an operation that he said was probably directed by Russia’s Federal Security Service (FSB), the main heir to the Soviet-era KGB.

An outspoken critic of Putin, Litvinenko fled Russia for Britain six years to the day before he was poisoned.

GELSEMIUM PLANT

Alexander Perepilichny, a 44-year-old Russian, was found dead near his luxury home on an exclusive gated estate outside London after he had been out jogging in November 2012.

Perepilichny had sought refuge in Britain in 2009 after helping a Swiss investigation into a Russian money-laundering scheme. His sudden death raised suggestions that he might have been murdered.

British police ruled out foul play despite suspicions he might have been murdered with a rare poison. An inquest into his death has yet to give a definitive conclusion as to how he died.

A pre-inquest hearing heard that traces of a rare and deadly poison from the gelsemium plant had been found in his stomach.

Perepilichny had enjoyed a large bowl of soup containing sorrel, a popular Russian dish. Russia denied involvement.

DIOXIN

Viktor Yushchenko, then a Ukrainian opposition leader, was poisoned during the campaign for the 2004 presidential election in which he stood on a pro-western ticket against the current Prime Minister Viktor Yanukovich.

He was poisoned with 1,000 times more dioxin than is normally present in the human body in 2004. His face and body were disfigured by the poisoning and he had dozens of operations following the poisoning.

Yushchenko subsequently won the presidency in a re-run poll after Ukraine’s Supreme Court, amid street protests dubbed the “Orange Revolution,” struck down the results of a first vote that gave victory to a pro-Moscow candidate.

He said he was poisoned while having dinner outside Kiev with officials from the Ukrainian security services. Russia denied any involvement.

HEAVY METALS

Vladimir Kara-Murza, a Russian opposition activist, says he believes attempts were made to poison him in 2015 and 2017. A German laboratory later found elevated levels of mercury, copper, manganese and zinc in him, according to medical reports seen by Reuters. Moscow denied involvement.