Martin Luther King Jr had dozens of extra-marital affairs and "looked on" as a fellow Baptist minister raped a female parishioner, according to astonishing new claims.

The FBI allegedly tape-recorded the civil rights activist on a number of occasions, revealing adulterous acts and a voracious sexual appetite, memos unearthed by his biographer David Garrow reveal.

In one troubling incident, King is alleged to have "looked on, laughed and offered advice" while a friend raped a woman in a Washington hotel room.

Martin Luther King Jr., gives his 'I Have a Dream' speech to a crowd before the Lincoln Memorial during the Freedom March in Washington, DC, on August 28, 1963. Photo / Getty Images
Martin Luther King Jr., gives his 'I Have a Dream' speech to a crowd before the Lincoln Memorial during the Freedom March in Washington, DC, on August 28, 1963. Photo / Getty Images

Mr Garrow says the FBI recordings were sealed for 50 years by a court order issued in 1977 and are not due to be fully available until 2027.

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They are currently held in a vault at the United States National Archive, he writes in the new edition of Standpoint magazine.

But Mr Garrow, who won a Pulitzer prize 30 years ago for his King biography, Bearing the Cross, described how he had unearthed written summaries of the FBI recordings which, if proved accurate, raise a wide range of concerns over the behaviour of King.

A key memo describes surveillance carried out by FBI officers at the Willard Hotel, near the White House, in January 1964.

King had invited friend Logan Kearse, a preacher at Baltimore's Cornerstone Baptist Church, back to his room with "several women parishioners of his church".

The conversation turned to "natural and unnatural sex acts", it is claimed. The memo goes on: "When one of the women protested that she did not approve, the Baptist minister immediately and forcibly raped her." King is alleged to have been in the room at the time.

Mr Garrow noted that FBI agents listening to the activities in the room - via microphones hidden in two lamps - did nothing to intervene to stop the alleged attack.

Surveillance continued the following night when King and a dozen other people took part in a sex orgy, it is claimed. Mr Garrow writes that the contents of the memos, some of which were written by deputy FBI director William Sullivan, would amount to a "painful historical reckoning" of King's reputation.

The author says that although he knew from research for his biography that King conducted affairs with ten or 12 women, the FBI files allege there were actually 40 or 45 extra-marital affairs.

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The FBI sent an incriminating tape and an anonymous letter to King calling him "abnormal" and warning him that "your adulterous acts, your sexual orgies" were "on the record for all time".

View of some of the leaders of March on Washington for Jobs and Freedom as they march with signs, Washington DC, August 28, 1963. Photo / Getty Images
View of some of the leaders of March on Washington for Jobs and Freedom as they march with signs, Washington DC, August 28, 1963. Photo / Getty Images

Nobel peace prize winner King was said to have told an aide over a wire-tapped telephone call that the FBI was "out to get me, harass me, break my spirit".

He was assassinated in Memphis in 1968 at the age of 39 by petty criminal James Earl Ray. If the authenticity of the recordings and the memos are verified it could lead to a root and branch evaluation of King's reputation in the MeToo era.

The FBI's surveillance was triggered by concerns that one of his key advisers was a "secret member' of the Communist Party, the Standpoint article alleges.

FBI director Edgar J Hoover was said to have been personally obsessed with the case and was convinced King's sexual activities would leave him open to communist blackmail.

King, who led the civil rights movement in the US from the mid-1950s, is remembered for his non-violent protests against segregation and his "I have a dream" speech from 1963.

He married Coretta Scott, an aspiring singer and musician, in 1953 and they had four children. Coretta died in 2006.