Princesses Beatrice and Eugenie are at the centre of new royal battle today because their father fears any children they have could be 'commoners'.
Prince Andrew wants to improve the status of his daughters by persuading the Queen and her heir Prince Charles to allow their future husbands to have earldoms.
'Otherwise his daughters' children will be commoners, defined as one of the ordinary or common people, as opposed to the aristocracy or to royalty,' a source told the Mail's Ephraim Hardcastle column.
It came as his 26-year-old daughter Eugenie was seen wearing a ring on her engagement finger as she left Mayfair private members' club LouLous with boyfriend Jack Brooksbank, 30, last night.
Royal convention means that all of the Queen's children enjoy the title HRH - but this is only passed on grandchildren born to the monarch's sons.
Princesses Beatrice, 28, is currently single after splitting with boyfriend of ten years Dave Clark while Eugenie, 26, has been in a relationship with nightclub boss Brooksbank for the past six years.
Mr Brooksbank's recent visit to Balmoral to meet her grandmother the Queen led to claims he asked Her Majesty for her blessing for a forthcoming engagement - but this was later denied by the couple.
If Beatrice or Eugenie have children with non-aristocrats or non-royals then their offspring will be without title, which Prince Andrew does not want, it was claimed last night.
The latest row came just weeks after the Queen found herself at the centre of a bitter feud between her two eldest sons over whether Princess Beatrice and Princess Eugenie should become taxpayer-funded working royals.
Prince Andrew has long demanded that his daughters, who receive no public funding, should be given more prominent roles - which Prince Charles has resisted.
Sources say the heir to the throne believes there 'is no public appetite' for more minor royals on the payroll and wants a 'streamlined' monarchy when he becomes king, with duties limited to himself, the Duke and Duchess of Cambridge, their children and Prince Harry.
But according to a newspaper, the brothers' battle took a twist when Andrew wrote to their mother demanding that his daughters, who are seventh and eighth in line to the throne, be given full-time royal duties and grace-and-favour apartments at Kensington Palace, instead of the smaller residence they currently share at St James's.
It was claimed the Queen was so unsettled by the letter that she passed it on to her private secretary, Sir Christopher Geidt, to deal with.
He reportedly raised the issue with Charles, who suggested a senior Government figure be brought in to convince Andrew that his daughters would be expected to make their own way in the world. There is no evidence this actually happened.
Both Buckingham Palace and Clarence House have declined to comment on the claims or indeed whether such a letter was written.
The report claimed a letter was drafted by Andrew's private secretary, Amanda Thirsk, but that the Duke of York ripped it up as he felt it was not strongly-worded enough and wrote his own version.
According to the Sunday Express, a source said: 'He believes his daughters are already being overshadowed by William, Kate and Harry and the situation will get worse as Prince George and Princess Charlotte get older.
'He has long argued that as the only "blood princesses" in [their generation of] the family, Beatrice and Eugenie deserve proper royal roles like their cousins, along with the same standard of accommodation at Kensington Palace. He fears they will be totally sidelined when the Queen dies.'
But a senior royal aide told the Mail that while Charles was fond of the princesses, he will not be swayed, adding: 'HRH has read the runes on this and doesn't believe there is public appetite for a large, unwieldy monarchy in the future - and that includes the York girls. It is one thing for [Charles's] brothers and sisters to continue with the valuable public work they do, but quite another for their children to be afforded the same role and status. It is unjustifiable in this day and age.'
The Queen's other children, Princess Anne and Prince Edward, did not seek royal titles or roles for their children. Anne's daughter Zara Phillips is an Olympic equestrian and son Peter runs a sports management firm.
Meanwhile Edward's wife Sophie, Countess of Wessex, recently spoke of how their children - James, Viscount Severn, eight, and Lady Louise, 12 - would need to find jobs outside 'the family firm' in the future. But Prince Andrew did take HRH titles for his daughters and their place in the monarchy has long concerned members of the royal household. The sisters, especially Beatrice, have a reputation for enjoying strings of lavish holidays.
Beatrice, 28, racked up 18 jaunts between December 2014 and December 2015, including several trips to the Caribbean and a stay on Roman Abramovich's superyacht in Spain. She has also left four jobs in five years. She stepped down from her last role in the summer to 'pursue her entrepreneurial ambitions', before holidaying in Greece and France.
Eugenie, 26, has been more circumspect, pursuing a career in the art world. But it recently emerged she may be given a cottage at Kensington Palace. It was suggested she would pay rent, but it was not clear how much she would to pay.
In 2010 she and Beatrice lost their 24-hour taxpayer-funded police protection after it was argued the estimated £500,000-a-year cost was unjustifiable. Neither Zara nor Peter Phillips have enjoyed the privilege.
