Joe Haines, who was press secretary to the former Labour prime minister, reveals details of the sensational request in a BBC television programme, How to be Prime Minister, to be broadcast next Sunday.
But Mr Haines told The Independent yesterday: "At the time, it was being reported that Amin was killing 7,000 people a week.
"Harold called me up to his study and said that he was very concerned about this. He asked me my view about killing Idi Amin, as he thought that was the only way of stopping the slaughter.
"I was against capital punishment, but I said that in this case we should make an exception. He was very concerned about Africa, and Amin, and he told me he was going to take the idea up with the Foreign Office.
"He was deadly serious about it, and he did take it up with the Foreign Office. I don't know who, but he told me later that the Foreign Office was scared out of its life.
"They said, 'We don't have anybody to do things like that.' Apparently, the Foreign Office has a strong line, that if we do that to them, they will do it to us."
James Bond's Secret Intelligence Service may have been licensed to kill, and the late Lord Wilson of Rievaulx evidently thought his services were available to him - as Anthony Eden had believed when he was Prime Minister.
It has often been suggested that Eden asked for the assassination of the Egyptian President Abdul Nasser during the 1956 Suez crisis. Sir Anthony Nutting, a Foreign Office minister at the time, said in 1985 that Eden had wanted Nasser "murdered".
Mr Haines said: "Harold did no more than Eden. They might have come from different eras, but Eden would have done that, and Harold did too. Amin was a madman indulging in the mass slaughter of his people."
But he was not assassinated by British secret agents, or anyone else. Having seized power in a 1971 coup, he was in turn ousted following a Tanzanian-backed invasion in 1979. Amin escaped to Libya, and later moved on to Saudi Arabia, where he was provided with a house, an income, and indeed still lives today. During his term of office, it was estimated that he was responsible for killing 300,000 of his own people.
"This was a madman," Mr Haines said. "I have a Christmas card from him to Harold, and it is signed, "'From Idi Amin Dada VC'. Mad."Reuse content