By Bruno Waterfield, the Daily Telegraph.

A secret Bulgarian unit known as "Service 7" was set up to "execute death sentences" on opponents to the Communist regime, secret files show.

One of the executioners, code-named "Piccadilly" is said to be the agent who carried out London's notorious killing of Georgi Markov in 1978 by using an umbrella to stab a poison filled pellet into the Bulgarian writer's leg.

The existence of the death squad, long denied by former officials, came to light after Alexenia Dimitrova, a Bulgarian journalist, discovered files accidentally released by Bulgaria's intelligence service had released under a new law declassifying former state security archives.

Bearing the acronym, OM for "ostri meropriatia" or "sharp measures", the files detail the kidnapping, discrediting and killing of Bulgarian émigrés in the 1960s and 1970s.

The records, classed as "top secret", described covert operations against people identified only by codenames such as "The Black", "Lackey", "Traitor", "Hamlet", "Betrayer" and "Widower".

In one memorandum in 1970, Angel Solakov, Bulgaria's interior minister, instructed agents to carry out killings to practise for the political assassination of Andreas Papandreou, the father of the current Greek leader, George Papandreou.

"We need to execute a death sentence. At first glance, it seems a tough and dirty job, but for us it is noble," he wrote. "I don't know whether one day we would be asked to liquidate for instance Papandreou. Now we get smaller tasks, but we should gain some experience."

When created in 1963 "Service 7", under the command of a Colonel Petko Kovachev, had only four officers but rapidly expanded, "in view of the increasing workload", to become a unit employing 39 agents within four years.

The killers were recruited from loyalists to the Communist regime.

The files show the agent, now thought to be a man called Francesco Gullino, was also involved in an operation in Italy against another Bulgarian émigré.

Another target, Traicho Belopopski, a former Bulgarian intelligence officer who had defected to Britain, narrowly escaped death by poisoning in the 1960s.

Offered Bulgarian salami by his father on a visit to London in the mid-1960s, Mr Belopopski, who was suspicious because knew the methods of his former employer, threw the sausage to a dog, which quickly died in agony.