His Britannic Majesty, or Her Britannic Majesty, depending on the sex of the monarch, is a formal, or official, term for the sovereign power of the United Kingdom in diplomacy, the law of nations, and international relations.
For example, in the Mandate for Palestine of the League of Nations, it was His Britannic Majesty who was designated as the Mandatory for Palestine.
Britannic Majesty is used in all British Passports, where the following sentence is used:
Her Britannic Majesty’s Secretary of State Requests and requires in the Name of Her Majesty all those whom it may concern to allow the bearer to pass freely without let or hindrance, and to afford the bearer such assistance and protection as may be necessary.
This is a reduced version of the wording on this British Passport, issued in 1979:
Her Britannic Majesty’s Principal Secretary of State for Foreign and Commonwealth Affairs Requests and requires in the Name of Her Majesty all those whom it may concern to allow the bearer to pass freely without let or hindrance, and to afford the bearer such assistance and protection as may be necessary.
Another use of the term is on envelopes used for official correspondence being sent abroad: instead of the familiar endorsement “On Her Majesty’s Service” used on mail sent within the UK, the endorsement “On Her Britannic Majesty’s Service” is used.
We are citizens of The United Kingdom of Great Britain and Northern Ireland…On our passports this is translated as Teyrnas Gyfunol Prydain Fawr a Gogledd Iwerddon in Modern Welsh, which, like Cornish and Breton is a remnant of the Brittonic or Old British tongue.. Remarkably however the term Prydain or Pretania has survived both Gaelic and English suppression, as well as the poor quality scholarship of Irish, Irish-American, Scottish and English partisan academics.
Our passports also translate The United Kingdom of Great Britain and Northern Ireland in Gaelic as Rìoghachd Aonaichte Bhreatainn is Eireann a Tuath , continuing the Roman adaptation of Pretania as Bhreatainn, while Gaelic also uses the ancient name of Alba for Great Britain, as a remnant of Albion.
So Elizabeth II remains, as she has always been, Her Britannic Majesty, and Queen of Pretania, our 6,000 British Isles.