Guests watching the service will be invited to join "a chorus of millions" to swear allegiance in the service led by Justin Welby.

The "homage of the people", revealed on Saturday by Lambeth Palace, is a new addition to the ancient ceremony.

But campaign group Republic called it "nonsense" and "offensive".

Speaking to the BBC News, the archbishop reiterated the oath was "an invitation; it's not a command".

The King held an audience with the Archbishop of Canterbury ahead of Saturday's Coronation

"In every Anglican service, every Christian service, it is normal for congregations to participate," he said.

"It's an invitation - so if you want to join in at this point, by all means do so.

"If you don't want to, that's fine. There's no drama to it."

Asked about some newspaper reports suggesting he had gone "rogue", the archbishop insisted the service had been a "huge, collaborative [with Buckingham Palace and the Cabinet Office] and very lovely process".

The full details of the Westminster Abbey service were published by Lambeth Palace over the weekend, including that the theme of the service will be "called to serve".

The public will be given an active role in the ceremony for the first time as they are invited to swear allegiance to the King in a "great cry around the nation and around the world".

This "homage of the people" will replace the traditional "homage of peers" - where hereditary peers swear allegiance to the new monarch.

While reading out the oath, the archbishop will call upon "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."

The oath will be followed by the sound of a fanfare.

The Archbishop of Canterbury will then proclaim: "God Save The King", and those willing will be asked to respond: "God Save King Charles. Long live King Charles. May the King live forever."

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

"This kind of nonsense should have died with Elizabeth I, not outlived Elizabeth II."