A Bulgarian defector who said on his deathbed that he had been stabbed with a poisoned umbrella was in fact murdered, Scotland Yard disclosed yesterday - probably by a platinum pellet smaller than a pinhead.

The exile, Georgi Markov, 49, a writer and dramatist who had broadcast antigovernment political commentaries to his homeland, died in a London hospital Sept. 11. Four days before that, he said, a man had jabbed him in the thigh with an umbrella near Waterloo Bridge, Markov told friends he was sure he had been poisoned.

The tiny pellet that presumably was used to poison him was found under his skin.

Officials said they had never before seen anything like this exotic instrument of death.

"It is diabolical that this sort of thing could happen in a London street," said Assistant Commissioner David Powis, operational chief of Scotland Yard's detectives.

Markov fled from Bulgaria in 1970. In recent years he had been broadcasting attacks on the Communist government to his homeland over the U.S.-run Radio Free Europe in Munich.

Scotland Yard does not know who killed Markov. Assistant Commissioner Gilbert Kelland told reporters:

"The world is the field in this case. Your guess is as good as mine."

The intelligence community here is convinced that the killer was an agent from the Soviet bloc, perhaps from the KGB, the Soviet secret police.

At first, intelligence sources here were puzzled by the long gap in time between Markov's defection and his death. Emigres marked for assassination, they said, are typically liquidated within six months.

But now the intelligence community knows Markov's fate was not an isolated incident. Last month, another Bulgarian defector and journalist, Vladimir Rostov, was hit in the back as he was mounting an escalator in the Paris Metro.

Unlike, Markov, Rostov did not become ill at once.On Tuesday in Paris, while two Scotland Yard detectives watched, a French surgeon removed from Rostov's back a pellet identical to the one found in Markov's right thigh.

The presumed fatal instrument is an ingenious device that leaves almost no trace. The pellet is an alloy of rare metals, 90 percent platinum and 10 percent Iridium. It is one-fifteenth of an inch in diameter. Two minutes holes, each 16 one-thousands of an inch deep, had been drilled into this pinhead, at right angles to each other.

Detectives here have found an unidentified substance in the holes of the pellet that was embedded in Markov's skin. This substance may turn out to be a poison, but the Yard said that it could take months to complete tests.

Commissioner Powis said that the pellets are a remarkable piece of microengineering, possibly performed by a skilled watchmaker. Preparing an unbrella tip to fire such a device must also have required precision work of a high order. Why the pellet that worked in London failed in Paris is a mystery.

Markov, who was sentenced in absense to 6 1/2 years in prison in Bulgaria, was married to an Englishwoman. His main job was broadcasting cultural news over the foreign service of the BBC, an essentially nonpolitical task.

But when Sofia refused to permit him to return to Bulgaria to see his dying father, Markov turned to the attack.

Over Radio Free Europe, once supported by the CIA, and the West German Deutsche Welle, he broadcast extracts from his unpublished memoirs. Among other things, he accused Bulgarian leader Todor Zhivkov and other high officials of corruption. Markov and Zhivkov ahd been friends before Markov's defection.

On Sept. 7, Markov suddenly fell ill with a high fever. He told his wife and others that he had been jabbed with an umbrella earlier that day by a man who said in a foreign accent, "I am sorry." The man with the unbrella then jumped into a cab and disappeared. Four days later, Markov died. He was 49.

Scotland Yard's experts were being congratulated last night for finding so tiny an object under the dead man's skin. Whether or not they ever discover precisely who killed Markov and with that substance, they have added a new chapter to the annals of assassination.