Markov originally worked as a novelist and playwright, but during 1969 he defected from Bulgaria, then a Stalinist state governed by President Todor Zhivkov. After relocating to the West, he worked as a broadcaster and journalist for the BBC World Service, the US-funded Radio Free Europe, and Germany's Deutsche Welle. He criticised the Bulgarian Stalinist regime many times by radio. It is speculated that, as a result of this, the Bulgarian government decided to dispose of him, and asked the KGB for help.
He was killed at this bus stop at Waterloo Bridge after a ricin-containing pellet was fired into his leg, most likely by the Bulgarian secret police.
Agents of the Bulgarian secret police, Darzhavna Sigurnost (Bulgarian: Държавна сигурност, abbreviated ДС), assisted by the KGB had previously made two failed attempts to kill Markov before a third attempt succeeded. On 7 September 1978 (the birthday of Todor Zhivkov), Markov walked across Waterloo Bridge spanning the River Thames, and was waiting at a bus stop on the other side, when he was jabbed in the calf by a man holding an umbrella. The man apologized and walked away. Markov would later tell doctors that the man had spoken with a foreign accent. The event is recalled as the "Umbrella Murder" with the assassin claimed to be Francesco Gullino, codenamed "Piccadilly".
Markov recalled feeling a stinging pain from where he had been hit, he assumed by the umbrella tip. When he arrived at work at the BBC World Service offices he noticed a small red pimple had formed and the pain from being jabbed had not stopped. He told at least one of his colleagues at the BBC about this incident. That evening he developed a fever and was admitted to a hospital where he died three days later, on 11 September 1978, at the age of 49. The cause of death was poisoning from a ricin-filled pellet.