David Cameron is related to the Queen through King William IV's illegitimate daughter, according to Debrett's, the self-proclaimed authority on the aristocracy.

The MP for Witney is William IV's great, great, great, great, great grandson.

The genealogy experts say he is the Queen's fifth cousin twice-removed.

In a further twist last night, it emerged that Mr Cameron's wife Samantha has Royal connections too.

Research by Cracroft Peerage has discovered that the 31-year-old is descended from Charles II and Nell Gwyn. The King's mistress was her great, great, great, great, great, great, great, great grandmother.

Mr Cameron is descended from Elizabeth FitzClarence - one of 10 children William IV had with his long-standing mistress, actress Dorothy Jordan.

Charles Mosley, Debrett's editor-in-chief, said Dorothy's relationship with the King, who ruled from 1830 to 1837, was 'a scandal'.

But he added: "That century was less prudish about these matters than we are. It was not a secret. William IV's illegitimate children by Dorothy Jordan were recognised."

The King's wife, Adelaide of Saxe-Meiningen, bore him no legitimate heirs and when he died the throne passed to Queen Victoria.

A spokesman for the MP said: "Mr Cameron is blissfully unaware of this. He hasn't looked into his background."

In some ways William IV was a moderniser, as Mr Cameron likes to style himself.

During his reign, Parliament passed the first Reform Bill, which abolished the most corrupt elements of democracy at the time. On the other hand, William IV was the last sovereign to sack a government - in 1834.

Of Mrs Cameron, Patrick Cracroft-Brennan said her lineage stretches back to the First Duke of St Albans, who was one of the illegitimate sons of Charles and Nell Gwyn.

"Mr Cameron will be the most aristocratic leader of the Conservatives for many years."