Having been asked why MPs and Peers are swearing oaths to the new Monarch, I did a bit of digging and was slightly surprised at the result.
Paragraph 8.25 of the current edition of Erskine May reads as follows
“8.25: In the event of the demise of the Crown, Parliament meets immediately, pursuant to the Succession to the Crown Act 1707 … and all Members of both Houses may again take the oath. There is no statutory obligation to take the oath in these circumstances, although it has been the custom of Parliament and continues to be obligatory in the House of Lords.”
Paragraph 25.7 states that:
“25.7 Under s 5 of the Parliamentary Oaths Act 1866 any Member of the House of Lords who sits or votes without having taken the oath or affirmed is subject to a penalty of £500. In addition to taking the oath or affirming when first taking their seats, all Members of the Lords must take the oath or affirm:
1. in every new Parliament; and
2. after a demise of the Crown.” [emphasis added].
The oddity is that s 5 Parliamentary Oaths Act 1866 (which also appears to apply to Members of the Commons) says nothing whatsoever about the demise of the Crown:
“5 Penalty for omission to take oath.
If any member of the House of Peers votes by himself or his proxy in the House of Peers, or sits as a peer during any debate in the said House without having made and subscribed the oath hereby appointed, he shall for every such offence be subject to a penalty of five hundred pounds, to be recovered by action in the High Court; and if any member of the House of Commons votes as such in the said House, or sits during any debate after the Speaker has been chosen, without having made and subscribed the oath hereby appointed, he shall be subject to a like penalty for every such offence, and in addition to such penalty his seat shall be vacated in the same manner as if he were dead.”
Despite the fact that there is, apparently, no statutory obligation to take the oath after a demise of the Crown, it seems to have become customary nonetheless, and as a former Commons Clerk I am inclined to conclude that the custom rests on nothing more than that golden oldie, “practice of the House”.
That view appears to be supported by the recent House of Commons Library research paper, The death of a monarch, which emphasises the differences in requirements for the two Houses:
“3.2 Parliamentary oaths on Demise of the Crown.
In the event of a Demise of the Crown, all Members of both Houses of Parliament may again take the oath of allegiance. Although there is no statutory obligation for either House to do so, it has become the custom. Erskine May states, however, that it ‘continues to be obligatory in the House of Lords’, as Lords Standing Orders state that the Oath of Allegiance ‘must be taken or solemn affirmation made by all members before they can sit and vote in the House […] after a demise of the Crown’.”.
In conclusion, if it has become the custom of the two Houses to swear afresh on the demise of the Crown, then that is what is going to happen – whether required by statute or not.