Prince Harry and Andrew are to keep their royal roles as Counsellors of the State after Charles extended the title to other royals.
The role allows members of the Royal Family to stand in for the King when he cannot fulfil official duties - and will now include the Princess Royal and the Earl of Wessex.
King Charles' decision to widen the pool of Royals that could represent him was announced in a signed message read to the House of Lords by the Lord Chamberlain, Lord Parker of Minsmere; the most senior official in the royal household.
Charles said the aim of the increase was to "ensure continued efficiency of public business when I am unavailable".
Counsellors of State, who can deputise for the monarch if he is overseas on an official trip or ill, include the Duke of Sussex, who lives in California after stepping down as a working royal, and the disgraced Duke of York.
Provisions for the counsellors are made under the Regency Acts 1937 to 1953, and those who can currently stand in for Charles include the Queen Consort, and the four most senior adults in the line of succession - Prince of Wales, Harry, Andrew and Princess Beatrice.
Speaking at the despatch box in the upper chamber, Lord Parker said he had the honour to present a message from the King "signed by his own hand".
The independent crossbencher told peers: "The message is as follows: 'To ensure continued efficiency of public business when I am unavailable such as while I am undertaking official duties overseas, I confirm that I would be most content, should Parliament see fit, for the number of people who may be called upon to act as Counsellors of State under the terms of the Regency Acts 1937 to 1953 to be increased to include my sister and brother - the Princess Royal and the Earl of Wessex and Forfar - both of whom have previously undertaken this role'."
There were approving shouts of "hear, hear" from the red benches at the end of the King's message being read out.
It comes after Labour Peer Viscount Stansgate, the son of the late left-wing firebrand Tony Benn, recently questioned the current list of royals able to officiate when the King is not available, given it included the Duke of York and the Duke of Sussex, "one of whom has left public life and the other of whom has left the country".
It is thought any changes to legislation to increase the number of counsellors would be completed later this year before the prospect of the King and Queen Consort going on overseas tours in 2023, which may coincide with the Prince and Princess of Wales also being out of the country.
Counsellors of State are authorised to carry out most of the official duties of the Sovereign like attending Privy Council meetings, signing routine documents and receiving the credentials of new ambassadors to the UK.
But certain core constitutional functions cannot be delegated, like appointing a prime minister or creating peers.
The message was also delivered in the House of Commons.
Raising a point of order, Commons Leader Penny Mordaunt said: "It may be of help if I inform the House that honourable members will have the opportunity to consider a response to His Majesty's gracious message ahead of the Opposition Day debate tomorrow.
"It may also help if I inform the House that there will be legislation relating to this message for the House to consider in due course.
"Should the House agree to the humble address as first business tomorrow, that legislation will provide a proper opportunity to debate the matter that has been raised."
