A centuries-old custom means the Duke of Westminster's first born child is not the one who will inherit his title and estate following his death.

Lady Tamara is the eldest child of Gerald Cavendish Grosvenor and his wife Natalia Phillips. Lady Tamara, 36, was married to Edward van Cutsem in 2004.

But despite being the first born, due to the rule of primogeniture, Lady Tamara will not assume the family's seat of Eaton or the billion-pound fortune because she is female.

Primogeniture is a principle that sees legitimate male heirs take precedence over their sisters when it comes to inheriting an estate - with the right of succession handed to the eldest son.

It is this rule which has seen Sir Gerald's dukedom pass exclusively to his third-born child, Hugh Grosvenor, at the age of just 25 - leapfrogging both his sisters, Lady Tamara and Lady Edwina.

Primogeniture has been traditional in England since the reign of William the Conqueror.

The rule in relation to the Royal family was scrapped under the Succession to the Crown Act, passed in 2013.

Removing the male bias from the succession line, the radical shake-up means Princess Charlotte is directly in line to the throne after Prince George.

Previously, under the ancient rules of male primogeniture, royal sons took precedence over their female siblings, even jumping ahead of first-born royal daughters.

But this same change has not been extended to succession for other titles.

During the commons debate of the bill there was talk the law could be changed further to include other titles so that daughters could inherit their father's estate.

Nick Clegg said at the time it was "not practical" to include such wide-sweeping changes and said the bill must be kept narrow so that it would be accepted by the other Commonwealth nations.

If there are no male heirs to which a title can pass, or if it cannot pass to a female in the line, it will then become extinct, according to peerage publishers, Debrett's.