On 7 September 1978 in London, Georgi Markov was struck by an umbrella in an attack widely believed to have been a political poisoning linked to the KGB. He died four days later. In the following year, Markov’s widow and older brother told the BBC programme Panorama that they had received warnings about a planned murder only months earlier from an unidentified caller.

"He felt a sharp jab in his thigh. When he turned around, there was a man behind him who apologised and picked up an umbrella." These were the recollections of Annabel Markov, speaking about her husband Georgi, the Bulgarian writer, journalist and dissident.

On 7 September 1978, Markov, then 49, was standing at a bus stop on Waterloo Bridge in central London when he was apparently attacked with an umbrella. He died in hospital four days afterwards. Doctors first believed he had severe blood poisoning, but investigators soon came to suspect a carefully planned political killing involving an advanced method that may also have been used against another Bulgarian exile.

In 1979, Markov’s widow and elder brother described to the BBC how a stranger had warned them months in advance that he would be poisoned.

Accounts from a former leader of a KGB assassination unit and a former Bulgarian secret-service colonel, both speaking publicly at the time, suggested that responsibility for the operation reached the highest circles of both the Bulgarian and Soviet states.

Shortly after Markov’s death, officers from Britain’s anti-terrorist branch found evidence that persuaded them he had been targeted in an extraordinary murder scheme. "At the Scotland Yard forensic laboratory, the chief scientific officer examined a tiny metal item recovered from Markov’s thigh," Panorama reporter Michael Cockerell said.

"Microscopic examination showed it was an exceptionally sophisticated pellet, smaller than a pinhead. Four minute channels ran through it, and forensic specialists believed these had held an almost immeasurably small quantity of a deadly poison."

Researchers at Porton Down, Britain’s secretive chemical and biological warfare research centre, concluded that ricin, an extremely lethal toxin, was likely responsible for Markov’s death. The programme reported that ricin had not previously been used in a murder in Britain.

It later became clear that another Bulgarian exile may have suffered a comparable attack 10 days before Markov was wounded. Vladimir Kostov experienced a sudden stabbing pain in his back while leaving the Paris Metro with his wife. He developed a serious fever lasting three days, but survived.

After learning about Markov’s death, Kostov arranged for an X-ray of his back, which showed a minute metal object. At a Paris clinic, a surgeon removed it while a French police officer, dressed in surgical clothing, observed the procedure.

"The surgeon had to proceed with great caution. He took out the metal pellet without making direct contact with it," Kostov told the BBC. The object was passed to a Scotland Yard detective and transported to England for examination.

Using a powerful scanning electron microscope, Scotland Yard metallurgists studied the pellet from Kostov’s back and found it matched the object recovered from Markov. "The two pellets had precisely the same weight," Cockerell said. "Their holes were positioned identically, and each was made from an unusual platinum-iridium alloy that is not rejected by the human body."

Toxicology experts at Porton Down detected ricin antibodies in Kostov’s tissue and believed he had lived because the pellet had contained too little poison.

The complexity of the method, together with signs that the attacks were linked, convinced Scotland Yard that an intelligence organisation acting on behalf of a state had been involved in Markov’s killing.

Before leaving Bulgaria, Markov had been a successful novelist and playwright with access to the country’s ruling circles. He defected from communist Bulgaria in 1969 and later settled in Britain, where he became a vocal critic of the Bulgarian authorities. He worked for the BBC World Service and Radio Free Europe, a broadcaster funded by the United States.

Annabel Markov said the family had been alerted to an intention to kill her husband. She told the BBC that an unknown man contacted Georgi in May 1978.

"They spoke Bulgarian all evening. From time to time Georgi would turn to me and say in English, ‘What this man is saying is extraordinary.’" Annabel said the caller claimed that Bulgaria’s political leadership had decided to kill him because of the critical broadcasts he was preparing for Radio Free Europe.

Markov’s brother Nikola supported the account, saying he had received a telephone call in September 1977. "A man called and told me the decision had been made to kill Georgi, to make him disappear without a trace," he said.

"At first I dismissed it. It sounded ridiculous and impossible. But he gave me details that were too specific to ignore. He said Georgi would certainly be poisoned at the first chance, and that some kind of poison or bacteria, though he did not know which, had already been sent to Western Europe."

After arriving in Britain, Markov remained well known in Bulgaria through his Radio Free Europe broadcasts. Each week, he described life among the country’s powerful elite. One series, called Life Behind the Curtains, referred to the small curtains fitted along the sides and rear windows of official black Mercedes cars. His programmes offered a sharp and often gossipy account of unchecked authority, including claims of favouritism, corruption and financial misconduct.

Bulgarian officials publicly insisted that Markov’s broadcasts had little effect. However, the BBC learned that Bulgaria’s president, Todor Zhivkov, was especially angered by a candid sequence of broadcasts titled My Meetings with Todor Zhivkov.

"We heard that he was extremely angry about Markov’s programmes concerning their meetings," said Kyril Panov, a Bulgarian broadcaster at Radio Free Europe. "Markov spoke about subjects that were effectively forbidden in Bulgaria—matters the regime preferred to hide from the public."

Zhivkov had governed Bulgaria since 1956 and maintained exceptionally close relations with the Soviet Union. He described the connection between the two countries as two lungs belonging to one body and supplied by the same blood.

Many observers believed the Soviet authorities had to have been involved in Markov’s murder.

Stefan Sverdlov, a former colonel in the Bulgarian secret police who defected in 1971, told the BBC in 1979: "Every department of Darzhavna Sigurnost, the Bulgarian secret service, is overseen by an adviser from the Russian KGB, sometimes called an uncle, who reports directly to Moscow. It is impossible to imagine this being done without Russian knowledge."

Nikolai Khokhlov, once the head of a KGB assassination group, said the method itself indicated Soviet participation. "The technique is almost absurdly sophisticated," he said. "That suggests the officers who planned the operation were trying to satisfy someone at the very highest level." Markov was killed on Zhivkov’s birthday.

Following the collapse of the Soviet Union, information emerged that the KGB had developed an umbrella capable of firing ricin-filled pellets into a target. Two KGB defectors later claimed that their agency helped with the killing, although they said the attack was carried out by an Italian criminal known by the codename Piccadilly.

No one has ever faced charges for Markov’s assassination. Bulgaria ended its investigation in 2013 because there were no identified suspects, while the inquiry remains open in the United Kingdom.

The operation may have served the Soviet bloc in ways beyond the death of a single critic. As Khokhlov told the BBC in 1979, it could create a powerful sense of fear among dissidents living overseas: the feeling that distance and Western security arrangements could not keep them safe.

He said: "It is like approaching someone from behind and whispering, ‘We can reach you whenever we choose. We can kill you.’"