In the typewritten letter, dating back to 1964, a deputy of J Edgar Hoover, the first director of FBI, attempts to blackmail King into committing suicide.
Using a shocking sexual slur, the author, identified as agent William Sullivan, draws on King's well-documented extra-marital liaisons.
FBI wiretapped King's homes and hotel rooms to gather evidence of his extra-marital affairs, which was recorded on a cassette that accompanied the letter.
An excerpt from the letter read: "Lend your sexually psychotic ear to the enclosure. You will find on the record for all time your filthy, dirty, evil companions, male and females giving expression with you to your hideous abnormalities.
"It is all there on the record, your sexual orgies. Listen to yourself you filthy, abnormal animal. You have been on the record — all your adulterous acts, your sexual orgies extending far into the past. This one is but a tiny sample. King, you are done."
The original 'suicide letter' was heavily censored when first released to public.
The entire letter has now been made public by Beverly Gage, a historian at Yale University, who discovered an uncensored copy of the letter in the National Archive when researching for a book on Hoover.
The letter concluded by urging King to commit suicide within 34 days.
"There is but one way out for you. You better take it before your filthy, abnormal fraudulent self is bared to the nation," read the letter.
King had apparently shown the letter to his advisers who confirmed that it was a work of the FBI in an attempt to blackmail him into suicide.
A 1976 Senate hearing into the working of intelligence agencies, including FBI, also concluded that the letter "clearly implied that suicide would be a suitable course of action for Dr King."
The letter was allegedly an act of revenge against King after he accused the FBI of failing to guard black people's security in the Deep South.