All Britons will be called on to swear allegiance to King Charles III at his coronation, an oath hitherto reserved for British nobility, in a move that has upset anti-royalists.

The office of the Archbishop of Canterbury, Justin Welby, who will lead the coronation ceremony on May 6 at Westminster Abbey, announced on Saturday that the traditional "Homage of Peers" – during which representatives of the nobility kneel before the king and pledge allegiance to him – would be scrapped.

Instead, the ceremony will include a "Homage of the People", with the archbishop calling on all people in the United Kingdom and other places where King Charles is the head of state to swear allegiance.

The archbishop will call on "all persons of goodwill in the United Kingdom of Great Britain and Northern Ireland, and of the other realms and the territories to make their homage, in heart and voice, to their undoubted king, defender of all".

The order of service will read: "All who so desire, in the abbey, and elsewhere, say together: I swear that I will pay true allegiance to Your Majesty, and to your heirs and successors according to law. So help me God."

British parliamentarians, but also Canadians since the British sovereign is their head of state, already swear allegiance to the monarch when they take office.

On Sunday, several elected representatives interviewed in the British media indicated that they will take the new oath during the coronation.

However, the plans to ask the public to pledge their allegiance to the king during the coronation have been branded "offensive, tone deaf and a gesture that holds the people in contempt" by an anti-monarchy group.

Graham Smith, a spokesman for Republic, which campaigns for the abolition of the British monarchy, said: "In a democracy it is the head of state who should be swearing allegiance to the people, not the other way around."

The anti-monarchist movement plans to demonstrate on Saturday.

The introduction of the new oath is one of the changes to the centuries-old ceremony, which the palace wants to evolve, particularly to reflect the country's diversity.

The last coronation was 70 years ago when Charles' mother, Queen Elizabeth II, was crowned.

She died in September aged 96.

At Saturday's ceremony, women bishops will take part for the first time, as well as representatives of non-Christian faiths.

Another new feature is that texts will be read out in the languages of the other nations of the United Kingdom: Welsh, Scottish Gaelic and Irish Gaelic. (AFP)