For most families, it is naturally a new baby’s first name – rather than its surname – which inspires fevered debate.

But not so in the aristocratic Mountbatten clan. The Mail on Sunday can reveal that the well-heeled dynasty, which includes the late Duke of Edinburgh, has been deliberating over the surname of its latest progeny – a baby boy, whose existence has, until now, not been widely known.

The child, called Theodore, was born more than a year ago and is the first-born son of Harry Mountbatten, the Earl of Medina and eldest son of the King’s cousin George, the Marquess of Milford Haven.

Theodore’s mother is Harry’s ex-girlfriend Bella Buchanan, 35, who is the granddaughter of the late presenter Nicholas Parsons, the legendary host of Radio 4’s Just A Minute who died five years ago.

As the only male Mountbatten of his generation, Theodore could have stood to inherit his father’s family’s lofty titles.

But the child was born out of wedlock – meaning, thanks to an archaic twist of fate, that those titles cannot be applied – and Bella was opposed to her son taking the Mountbatten name after her relationship with Harry ended while she was pregnant. 

Official documents show that Theodore has had two different surnames during his short life to date. 

His original birth certificate was issued featuring his mother’s surname, Buchanan – which meant his first passport was in the name of Theodore Buchanan.

Baby Theodore was born more than a year ago to Harry Mountbatten, the Earl of Medina and his ex-girlfriend Bella Buchanan granddaughter of the late broadcaster Nicholas Parsons

Bella, pictured with the legendary host of Radio 4’s Just A Minute, was opposed to her son taking the Mountbatten name after her relationship with Harry ended while she was pregnant

Now George Mountbatten, the Marquess of Milford Haven (pictured with his wife Claire) has intervened personally – insisting his grandson must officially be known as a Mountbatten

It is understood this was, in part, because the 33-year-old Earl had parted ways from Bella after two-and-a-half years together, and before Theodore was born.

But within months, the Marquess of Milford Haven had intervened personally – insisting that his grandson must officially be known as a Mountbatten.

A source said: ‘The Marquess told Harry that the first-born son of his own first-born son must be a Mountbatten. Harry agreed.’ 

Bella, however, initially refused.

The source added: ‘Bella was perfectly happy with Buchanan. She didn’t want a title from the family when she had been left single and broken-hearted.

‘Harry wasn’t there for the birth, so she felt he didn’t have the right to call the shots. 

‘But the Marquess is a persuasive man and he is extremely kind and a doting grandfather to Theo, so eventually Bella relented for his sake.’

An updated birth certificate, also seen by this newspaper, was issued in October to show the boy had officially been renamed Theodore Mountbatten. 

The name was adopted in 1917, during the First World War, when Prince Louis of Battenberg, the current Marquess’s great-grandfather, anglicised the family name in response to anti-German sentiment in Britain. Pictured: The family’s coat of arms

Bella’s change of heart meant a ‘dash to the printers’ to change the name on the order of service for Theo’s christening in October.

Sources say the Marquess, 64, who sold his comparison website uSwitch for £210 million in 2006, is a regular visitor to the baby, who looks ‘every inch a Mountbatten’, with the same blue eyes and blond hair as his father and grandfather.

Amid the debate over his name, society bible Tatler left him out of a recent article about the family’s grandchildren.

The source added: ‘Theo has been recognised by the Mountbatten family, though he has been kept out of the public eye. 

‘Even when Prince Harry’s former girlfriend, Cressida Bonas, attended Theodore’s christening, Tatler’s society pages remained silent.’

Harry lives in a multi-million-pound house in Knightsbridge while Bella and Theo live nearby in a rented home.

The Mountbatten family has form for playing with names. The title was adopted in 1917, during the First World War, when Prince Louis of Battenberg, the current Marquess’s great-grandfather, anglicised the German-sounding family name in response to anti-German sentiment in Britain.

Bella works part-time with Four Mamas, a company supporting women through pregnancy and motherhood. She declined to comment.

Last night, when asked about Theodore, Harry said: ‘Like all of his cousins, he is surrounded by our warm family that love him more than anything. I can’t wait for all our adventures in the future.’