Anatoli Kuznetzov, 49, the best-selling Soviet author who defected to Britain in 1969, died Wednesday at Whittington Hospital after suffering an apparent heart attack at his home.

Best known for his 1966 book "Babi Yar," which recounted the Nazi massacre of thousands of Russian Jews near Kiev, Mr. Kuznetsov was granted asylum in July 1969 when he arrived in Britain on a trip financed by the Soviet Writers Union.

Mr. Kuznetsov, who suffered a heart attack nine months ago, once said he defected because his writings were being distorted by Soviet censors, who he charged had turned him into an "ideological pot-boiler."

"I could no longer write, no longer sleep, no longer breathe," he said.

In recent years, Mr. Kuznetsov contributed weekly broadcasts to radio Liberty, a Munich-based radio station whose programs are beamed to the Soviet Union and Eastern Europe.

Mr. Kuznetsov's defection came as a surprise in Soviet liberary cricles, where it has been assumed he was a loyal, if a bit testy, member of the Soviet establishment.

He was given permission to travel to England in 1969 to research a book on the life of Lenin. But his purpose was to defect - a move for which he prepared by sewing 35 mm negatives of his writings into the lining of the clothing he brought with him.

He left behind his wife, a son and a woman who claimed to be his pregnant mistress.

The Soviet government's reaction to Mr. Kuznetsov's defection was harsh. It said he was "stripped forever" from the rolls of Soviet Russian writers and labeled him a traitor to the motherland.

Mr. Kuznetsov's response was equally bitter. He gave up the use of his name, calling himself simply "A. Anatol," and sent several harsh letters to Soviet authorities explaining why he left.

Born in 1929 in Kiev, the Ukraine, Mr. Kuznetsov began writing as a teen-ager. His first articles were published in 1949 in a factory newspaper.

His other books, "Sequel to a Legend," The Life of a Young Man," "Selenga," "At Home" and "Fire" were based on research conducted during various construction jobs he held in his youth.

He became a Communist party member in 1955, and shortly before his defection was named to the editiorial panel of the monthly youth magazine. His name was on the masthead for only one edition before he defected. CAPTION: Picture, ANATOLI KUZNETSOV, 1969 Photo