Bulgaria's President Peter Stoyanov has told the BBC that the assassination of the dissident writer Georgi Markov was one of the darkest moments in the history of the former communist regime.

Speaking on the 20th anniversary of Mr Markov's death, President Stoyanov said authorities would continue to investigate the case even though the former secret service in Bulgaria had destroyed evidence.

Georgi Markov, a BBC World Service journalist and strong critic of the communists, died in London after apparently being injected with poison from the tip of an umbrella.

KGB: Soviet secret service believed behind murder

Nobody has ever been charged with the murder, one of the most infamous of the Cold War period, but it is widely believed the Bulgarian secret service and the KGB were behind it.

The 49-year-old was an acclaimed novelist and playwright in Bulgaria prior to his defection to the West in 1969.

He was particularly known for his harsh criticism of the autocratic rule of the communist leader, Todor Zhivkov, and his broadcasts were seen as providing inspiration to the dissident movement in Bulgaria.

His death followed an incident at a bus stop on route to the Bush House headquarters of the BBC's World Service when Mr Markov experienced a sudden stinging pain in the back of his right leg.

Though in pain, Mr Markov continued on his way to work, but by evening had developed a high fever and died three days later.

After weeks of research and experimentation to identify the substance that had been used to cause the death, a coroner ruled that Mr Markov had "been unlawfully killed".