Revealed: how Cameron is related to the Queen

But after three months of playing down his aura of aristocratic privilege, the heir apparent to the Conservative Party leadership was dealing with a revelation yesterday that will give jibes about his poshness an extra edge.

The plummy-voiced son of an Oxfordshire stockbroker is the great-great-great-great-great grandson of King William IV and thus a distant relative of the Queen.

The noble lineage of Michael Howard's expected replacement also comes with a faint whiff of early 19th-century scandal - Mr Cameron's blue blood stems from the king's affair with a leading actress via one of their 10 illegitimate children.

The evidence that establishes a bloodline between the Queen and her would-be Tory prime minister has been uncovered by Debrett's Peerage and Baronetage, the genealogical bible of the upper crust.

Charles Mosley, the editor-in-chief of Debrett's, said: "It will at least make it easier for Mr Cameron to break the ice if and when he meets the present sovereign as her prime minister.

"I don't know what it says about a meritocratic Britain. There is no doubting that David Cameron got off to a privileged start but it is part of the haphazard tapestry of life that he finds himself as a fairly close relative to the monarch."

The Oxford-educated Tory politician is fifth cousin twice removed of the Queen, according to the genealogists.

The link has its roots in the relationship - an open secret in the royal court - between William IV, the last member of the German House of Hanover to rule Britain, and Dorothy Jordan, a leading actress in late 18th-century and early 19th-century London. One of their children, Elizabeth, born in 1801, is the forebear of Cameron.

Despite being illegitimate, the children of Mrs Jordan (who was unmarried but used the honorific "Mrs" to give her more respectability on the stage) were officially recognised by their father and given the name FitzClarence.

William IV's wife, Princess Adelaide of Saxe-Meiningen, bore him no surviving heirs so the crown passed to his 18-year-old niece, who became Queen Victoria, the present Queen's great-great-grandmother.

A bewildering assortment of marriages between landed gentry and assorted aristocrats, including the 18th Earl of Erroll and the 5th Earl of Fife, places Mr Cameron - who recently displayed his populist credentials by urging listeners of a London community radio station to "keep it real" - firmly in the lineage of those born to rule by birth.

Mr Mosley said: "If William IV had been allowed to marry Dorothy Jordan, then David Cameron would be more closely related to the monarch than he is to the present Queen."

All of which caused mild consternation in the Cameron camp as the Conservatives prepare to unveil their new leader, supposedly the standard bearer of a modern, merit-based Britain.

A Cameron aide said: "I know for a fact that Mr Cameron is blissfully unaware of this. Journalists seem to know far more about his family history than he does.

"He hasn't looked into his background, but he's said as a passing joke that he's going to. I can't confirm or deny it."

If he chooses, the would-be Tory leader can point to his great-great-great-great-great-grandfather as a moderniser of sorts.

William IV, who spent most of his career in the Royal Navy and had not expected to succeed to the throne in 1830, helped to ensure that the 1832 Reform Act - which extended the franchise and abolished rotten boroughs - was passed. During his seven-year reign, the poor law was reformed and slavery abolished.

But Cameron cannot go too far in touting his ancestor's democratic credentials. William IV was also the last British monarch to appoint a prime minister against the will of Parliament.