Jonathan Eig made the discovery while conducting research for his forthcoming book, King: A Life, in the archives of Duke University in Durham, North Carolina.

Eig found that in a 1965 interview with Playboy magazine, King was erroneously quoted as accusing X of “fiery, demagogic oratory” – words which have contributed to perceptions of a deep divide between the two men.

“I think [the] historic reverberations are huge,” Eig told the Washington Post. “We’ve been teaching people for decades, for generations, that King had this harsh criticism of Malcolm X, and it’s just not true.”

The interview between the journalist Alex Haley, then 43, and King, then 36, came at the high-water mark of the civil rights movement and was the longest published interview King had then given. The interview has been republished countless times, contributing to portrayals of a fractious relationship between the two leaders.

Eig said his discovery “shows that King was much more open-minded about Malcolm than we’ve tended to portray him”.

Eig found the misquotation – and several others – in 84 typed pages of what appears to be the unedited transcript of the full interview between Haley and King.

On page 60, Haley asks: “Dr King, would you care to comment upon the articulate former Black Muslim, Malcolm X?”

King responds: “I have met Malcolm X, but circumstances didn’t enable me to talk with him for more than a minute. I totally disagree with many of his political and philosophical views, as I understand them. He is very articulate, as you say.

“I don’t want to seem to sound as if I feel so self-righteous, or absolutist, that I think I have the only truth, the only way. Maybe he does have some of the answer. But I know that I have so often felt that I wished that he would talk less of violence, because I don’t think that violence can solve our problem. And in his litany of expressing the despair of the Negro, without offering a positive, creative approach, I think that he falls into a rut sometimes.”

King’s words appeared differently in the published interview.

While the beginning of King’s remarks are identical to the transcript, in the published interview, King’s quote ends as: “And in his litany of articulating the despair of the Negro without offering any positive, creative alternative, I feel that Malcolm has done himself and our people a great disservice.

“Fiery, demagogic oratory in the Black ghettos, urging Negroes to arm themselves and prepare to engage in violence, as he has done, can reap nothing but grief.”

The line “I feel that Malcolm has done himself and our people a great disservice” does not appear anywhere in the 84-page transcript, the Post said.

In question-and-answer interviews, it is common for journalists to make slight edits, including shortening long answers or removing excessive pauses. But it is standard practice not to change the intended meaning of quotations, and to note any changes.

Eig said Haley’s treatment of the King interview amounted to “journalistic malpractice”.

“I believe that what we know about this incident, and others that we know about Alex Haley, should prompt us to scrutinize everything he’s written, including the autobiography,” Eig said.

The Autobiography of Malcolm X: As Told to Alex Haley was published in October 1965, nine months after X was assassinated in New York City.

Haley died in 1992, aged 70. His work has been questioned elsewhere. Most visibly, the author has been accused of historical inaccuracy and plagiarism in his most famous work, Roots: The Saga of an American Family.