Much has been said of the humble roots of the Duchess of Cambridge, with coal miners, a road sweeper and even a prisoner among her ancestors. Now, it has been found that Catherine is related to one of Britain's grandest families and can count a prime minister, earls and countesses among her kin.

When the duchess' first child is born, the third-in-line to the throne will not be the only blue blood in her family tree, the Telegraph reported.

The duchess has been informed of her aristocratic heritage.

A previously unpublished side of the duchess' family tree uncovered her link to William Petty FitzMaurice, the First Marquess of Lansdowne, who served as the prime minister from 1782 to 1783. He is also known for negotiating the end of the American War of Independence.

His link to the duchess was discovered by Michael Reed, a history teacher from Melbourne, after he asked his class to choose one of three well-known figures -- David Beckham, American rapper 50 Cent or the duchess -- and to look at their ancestry using an online family tree programme.

While assisting one of his pupils, Reed found a link from the Middletons to Lady Barbara Bullock, née Lupton, whom he identified as the duchess' second cousin.

Further research found that in 1917, Barbara Lupton married Christopher Bullock, a Cambridge scholar and descendant of William Petty FitzMaurice.

Bullock served as Winston Churchill's principal private secretary in 1919, and later became permanent under-secretary at the British Air Ministry from 1931 to 1936.
Previous research linked the duchess to a family of miners in Durham, a west London road sweeper and an inmate of Holloway Prison.

Reed sent his findings to the duchess at St James' Palace, following which the duchess asked her assistant private secretary to thank him for his discovery.