Millions of people will be asked to say a pledge of allegiance to the King when he is crowned on Saturday.
For the first time in history, the public will be given an active role in the coronation, with the Archbishop of Canterbury calling on those watching or listening to the event to take part in a "great cry around the nation and around the world of support for the King".
People watching the ceremony will be invited to say the words: "I swear that I will pay true allegiance to Your Majesty, and to your heirs and successors according to law - so help me God".
Royal biographer Angela Levin said "I am a huge believer in the monarchy but I am not going to swear "true allegiance." Step too far I'm afraid."
Levin recently released a book on Queen Consort Camilla and is known for her criticism of Prince Harry and Meghan Markle.
Many other supporters of the royal family also criticized the decision.