The 20-page document, dated 12th March - three weeks before the civil rights leader was assassinated in Memphis - is included in the latest trove of government files about the former President's murder , which the National Archives released on Friday.
Many of the allegations contained in the records appear to be reports of private conversations between others, or hearsay reported third-hand.
One of several allegations was the result of "a very responsible Los Angeles individual in a position to know" that claimed Dr King had a mistress in California with whom he fathered a child.
Dr King is also accused in the files as having links to the Communist Party, setting up a "tax dodge" with his Southern Christian Leadership Conference organisation and having affairs with at least four women, including folk singer Joan Baez.
It also warns that despite Dr King's history of calling for peaceful protest, "the combined forces of the communist influence and the black nationalists advocating violence give the 'Washington Spring Project' a potential for an extremely explosive situation".
Dr King was assassinated before the planned march.
Although the FBI's allegations against Dr King have been well known for years, the newly released report offers insight into the picture that was created of the revered leader as he campaigned for civil rights.
There is no evidence that any of the claims in the report were verified and it is not clear why the files were kept secret for almost 50 years.
