A separate study into Mr McCain's ancestry has now found that the 71-year-old Arizona senator has the same royal lineage.

Senator McCain was the surprising Republican victor in the New Hampshire primary last week. The former navy pilot and Vietnam veteran described his Scottish credentials in his autobiography, Faith of My Fathers.

He wrote that "the McCains, bred to fight as Highland Scots of the Clan McDonald, arrived in America shortly after it gained independence".

Gary Boyd Roberts, a senior research scholar at the New England Historic Genealogical Society, claims that both Mr McCain and Mr Obama can trace their ancestry to Isabel, an illegitimate daughter of William the Lion.

Mr McCain's family was traced back to his four-times great-grandparents, Hugh and Agnes McCain, who emigrated to Caswell County, North Carolina, in 1778.

Mr Obama's ancestor, Edward FitzRandolph, emigrated to America more than 100 years earlier.

From the London Telegraph