The change will also affect Prince Andrew, as he was in a position to fill in for Charles if the monarch was unable to fulfil his duties.
The "slight to Harry and Andrew", was labelled as "necessary" by royal biographer Angela Levin, with the change being announced in a signed message to the House of Lords by the Lord Chamberlain, Lord Parker of Minsmere, the most senior official in the royal household.
Charles stated that the aim of the change was to "ensure continued efficiency of public business when I am unavailable". The King wants to ensure only working royals are Counsellors of State, which would change the current list dramatically.
Under the 1937 Regency Act, all Counsellors of State are the four highest candidates in the royal line of succession who are over the age of 21, at the moment they are: Prince William, Prince Harry, Prince Andrew and Princess Beatrice.
The King's wife, Queen Consort Camilla, is also able to fill in for Charles, even though she is not in the line of succession.
With the addition of Princess Anne and Prince Edward into the Counsellors of State, it's very unlikely Charles would ever rely on Prince Harry or Prince Andrew to step up if he, the Queen Consort or the Prince of Wales were unavailable.
"Of course, it is a slight to Harry and Andrew – but there's a good reason and it's necessary," Angela told MailOnline.
She continued, saying the move meant Harry and Andrew were "taken off the list in a very diplomatic way" that would be more practical for Charles.
"Harry and Meghan would be absolutely furious with the decision," she added. "But he lives in California, he's stopped being a working royal, so why should he [be a Counsellor of State]. It isn't about Harry, but instead what the King needs."
She added that if Andrew was to fill in for the King "the public would be absolutely furious", labelling Charles' move as "very sensible".
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