Patrick Cracroft-Brennan, editor of Cracroft's Peerage and one of the UK's leading experts in peerage law, in a recent addition to Cracroft Peerage's entry on the Dukedom of Sussex writes:
'The Duke and Duchess of Sussex issued a statement shortly after their son's birth that he would not bear any courtesy titles, and would be known as "Master Archie Mountbatten-Windsor". This is a complete break with tradition and with the laws governing peerages. Until such time as this is confirmed by a Royal Warrant we will continue to refer to him as the Earl of Dumbarton'.
He was, as son of a duke, BORN with the rank of a duke of in the Peerage of the United Kingdom, and is legally 'Lord Archie' Mountbatten-Windsor.
It was expected that Archie would be styled as Earl of Dumbarton, which is Prince Harry's earldom, created with the Dukedom of Sussex and Barony of Kilkeel in May, 2018.
A Palace statement announced that no courtesy titles would be borne by the seventh in line to the throne, and that he will be KNOWN as Master Archie Mountbatten-Windsor.
It will be interesting to see how the College of Arms handles the case of 'Master' Mountbatten-Windsor. The eldest son of a duke by custom uses one of his father's peerages as a 'courtesy' title, but like his younger brothers he is also 'Lord' [Christian name] Surname, and was born so, and will be recorded as such at the College of Arms.
In cases where a person succeeds to a peerage from a cousin or relation other than a parent then the siblings of that new peer do not automatically assume courtesy titles, and apply to the Crown to be so granted. Claims are reviewed and when approved the Crown Office issues the following:
'Her Majesty has been graciously pleased by Warrant under her Royal Signet and Sign Manual bearing the date …. to ordain and declare that …..(name of person seeking courtesy title) .. shall henceforth have, hold and enjoy the same title, rank, place, pre-eminence and precedence as the children of a ….Duke/Marquess/Earl/Viscount/Baron... as would have been due to them had their father … (his name)… succeeded to the title and dignity of Duke of …….and to command that the said Royal Concession and Declaration be recorded in Her Majesty's College of Arms.'
Archie Mountbatten-Windsor, being born the son of Duke, was born Lord Archie Harrison Mountbatten-Windsor as a legal entity. The 'courtesy' title Earl of Dumbarton may have been declared withheld or not to be used, but on legal documents will the Queen's latest great-grandchild be named as Lord Archie? My mole in the College of Arms certainly thinks so.