Kate could effectively be given all of the powers of the monarch under legal guidelines.
The Regency Act 1937 provides for the eventuality that a monarch is incapable of ruling for health reasons, or because the sovereign is not yet of age.
It orders that a Regent should perform the royal functions if “the Sovereign is by reason of infirmity of mind or body incapable for the time being of performing the royal functions or that they are satisfied by evidence that the Sovereign is for some definite cause not available for the performance of those functions”.
Also outlined in the legislation are provisions for the event of an heir being under the age of 18.
Queen Elizabeth II PA
The decision must be made by at least three of the wife or husband of the Sovereign, the Lord Chancellor, the Speaker of the House of Commons, the Lord Chief Justice of England and the Master of the Rolls.
The Act requires that the Regent should be the next person in the line of succession who was either over the age of 21, a British subject domiciled in the UK and capable of succeeding to the Crown under the terms of the Act of Settlement 1701.
Prince George of Cambridge, should he ascend to the throne prior to his 18th birthday on 22 July 2031, is the first person in the present line of succession who would require a regency and legal guardianship until he turns 18.
According to the Regency Acts as currently in force, should this occur, his legal guardianship would be vested in his mum Kate, Duchess of Cambridge.
As legal guardian, Kate would effectively wield the constitutional authority of the monarch until George becomes of age.
This would only happen in the heartbreaking event that the Queen, Prince Charles and Prince William all die before George turns 18.
George is currently just eight years old, so such an eventuality is highly unlikely.
If Kate were unable to carry out the duties of legal guardian, they would then revert to the sitting Regent.