"Good intelligence, combined with the arrests of Sikh and Kashmiri extremists, was believed to have frustrated plots to attack Rajiv Gandhi during his state visit," Cambridge historian Professor Christopher Andrew has said in his book The Defence Of The Realm.
The book, the first official history of MI5, says that after Operation Bluestar in the Golden Temple in June 1984, there was sudden emergence of Sikh extremism, particularly in the UK.
"Sikh extremism, which suddenly emerged in the UK as a major threat during the summer and autumn of 1984, was put at the top of the list of current terrorist threats in mainland Britain in 1985-86 annual report by MI5 Director General Tony Duff," the book says.
Rajiv Gandhi was killed by a LTTE suicide bomber at an election rally in Sriperambudur in Tamil Nadu in May 1991. At least 14 other people, including the attacker, were killed in the blast.
The book also says that under Indira Gandhi, India was described as the "arena for more KGB" active measures than anywhere else in the world. "Oleg Kalgunin, who became head of counter- intelligence in KGB foreign intelligence in 1973, remembers India as a model of KGB infiltration of a Third World government," it adds.