Mr Obama wrote in his memoir, Dreams From My Father, that his grandfather "was placed in a detention camp" after being reported as a subversive "to the white man" by a rival land-owner. After his release, "he was very thin and dirty", he "had difficulty walking, and his head was full of lice," wrote Mr Obama, who was told the story by relatives during his first visit to Kenya in 1988.

Mr Onyango's third wife, Sarah, has claimed that her late husband had his testicles crushed and his nails and buttocks pierced as jailers at Kamiti Prison outside Nairobi sought information on the rebels. Mrs Onyango, who is now 90, said Mr Onyango still bore scars from his treatment when he died in 1979, and that the story tarnished her step-grandson's view of the British. The President "has never believed the British do anything for a common good, rather than their selfish interests," she told an interviewer in 2010. "He said the whole act sounded barbaric. He wondered why the British never respected African culture."

Mr Obama denies this, yet the story has over the past four years been cited as an explanation for his snub of then-prime minister Gordon Brown, his early reluctance to talk about the "special relationship" and his decision to return a bust of Churchill that George W. Bush had displayed in the White House.

Zablon Okatch, who worked with Mr Onyango as a servant for American diplomats after his supposed detention, told Maraniss: "Hussein was never jailed. I know that for a fact. It would have been difficult for him to get a job with a white family, let alone a diplomat, if he once served in jail".

Dick Opar, a former senior Kenyan police official, said he "would have known" if Mr Onyango had been detained. "People make up stories," said Mr Opar. "If you get arrested, you say it was the fight for independence, but they are arrested for another thing".

Even one of Mr Onyango's daughters, Auma Magak, told Maraniss: "He was not detained", suggesting that he had once been kidnapped by thugs and the story had been twisted through the generations.

Maraniss also notes that no records exist of the detention and that within a year of his supposed imprisonment, Mr Onyango's son, Barack Obama senior, was accepted into a prestigious boarding school in western Kenya.

The alleged inaccuracy in Mr Obama's backstory is one of several noted by Maraniss, who also discloses the true identities of several girlfriends that the President compressed into one character in his 1995 memoir.

A White House spokesman declined to comment.