Counsellor of State
The counsellors of state are senior members of the British royal family to whom the monarch (Elizabeth II since 1952), can delegate and revoke royal functions through letters patent under the Great Seal, to prevent delay or difficulty in the dispatch of public business in the case of their illness (except total incapacity) or in their intended or actual absence from the United Kingdom.
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Counsellors of state may carry out "such of the royal functions as may be specified in the Letters Patent". In practice, this means most of the monarch's official duties, such as attending Privy Council meetings, signing routine documents and receiving the credentials of new ambassadors to the Court of St James's. However, by law, counsellors of state cannot grant ranks, titles or peerages. They also, by the terms of the letters patent, cannot deal with a number of core constitutional functions, such as Commonwealth matters, the dissolution of Parliament (except on the monarch's express instruction) and the appointment of Prime Ministers.
Royal functions are to be exercised jointly by the counsellors of state or by such number of them as is specified in the letters patent under the Great Seal and subject to any other conditions within. However, there is a legal presumption that counsellors of state should act jointly and, as such, at least two are needed to act, with the absence of one possibly risking a legal challenge.
Counsellors of state are the monarch's spouse and the next four people in the line of succession who meet the following specifications; they must be British subjects of full age (21, but 18 for the heir apparent and presumptive) who are domiciled in the United Kingdom and not disqualified from becoming monarch. During a regency, the next four eligible people in the line of succession after the regent may be counsellors (and the monarch's spouse).