Writer Georgi Markov became the most famous victim of the Cold War when he was assassinated on Waterloo Bridge in 1978.
Now known as the 'Umbrella murder', it is thought to be the work of the Bulgarian secret police assisted by the Russian KGB.
Mr Markov had defected from his communist homeland to the West in 1969 and had started a new life in London working for the BBC World Service.
On 7 September 1978 he was waiting at a bus stop on Waterloo Bridge on his journey to work at Bush House in central London.
Feeling a sharp pain on the back of his right leg he looked around and saw a man picking up an umbrella off the ground. This man apologised and then got into a cab and left.
Once at work Mr Markov noticed a red mark on his calf and was taken to hospital after developing a high temperature. He died on 11 September.
A postmortem resulted in the discovery of a small 1.52mm platinum pellet with two small holes drilled through it.
Inside it, scientists found traces of the poison ricin, a protein extracted from the castor bean for which there is no known antidote. The pellet is now on display at the Crime Museum at New Scotland Yard.
Since the murder the prime suspect has been named as Francesco Gullino, known by the codename 'Picadilly.' A KGB defector has also admitted involvement in organising the plot and supplying the weapon - which he claimed was a small pen-like device and not an umbrella - and the poison.
But no one has ever been charged and many of the original files in Bulgaria have been destroyed.
Markov began his career as a chemical engineer but published books, short stories and plays in Bulgaria throughout the 50s and 60s.
He left the country in 1969 after several of his works were banned by the Communist regime. In 1972 he was declared a traitor and sentenced to six years imprisonment in his absence.
It is thought he aroused the hatred of Communist Party Chairman Todor Zhivkov with a series of broadcasts on Radio Free Europe, which had a large audience in Bulgaria. The date of the assassination was Zhivkov's birthday.
Markov is bured at Saint Candida and Holy Cross Churchyard cemetery in Whitchurch Canonicorum, Dorset, England. The epitaph on his gravestone reads: 'In Memory of Georgi Ivanov Markov Novelist & Playwright Most dearly beloved By his wife Annabel His Daughter Sasha His Family & his Friends Born Sofia 1. 3. 39 Died London 11 .9. 78 In the Cause of Freedom.'
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