The St Edward's Crown has been removed from the Tower of London to be resized for the King ahead of the Coronation.
Buckingham Palace said the centrepiece of the Crown Jewels had been taken to allow for modification work to begin before the ceremony on May 6.
The movement of the priceless crown was kept secret until it had been safely delivered.
Versions of the St Edward's Crown are thought to have been used at the moment of coronation for British and English monarchs since the 13th century.
The current crown was made for Charles II in 1661, as a replacement for the medieval crown, which had been melted down in 1649.
The original was thought to date back to the 11th-century royal saint Edward the Confessor, the last Anglo-Saxon king of England.
It is the St Edward's Crown that appears in the royal coat of arms of the United Kingdom, the Royal Mail logo and in the badges of the armed forces.
In a statement, Buckingham Palace said on Saturday: "St Edward's Crown, the historic centrepiece of the Crown Jewels, has been removed from the Tower of London to allow for modification work to begin ahead of the Coronation on Saturday May 6 2023."
The coronation will take place in Westminster Abbey, eight months after the monarch's accession and the death of the Queen.
It is understood that the ceremony will include the same core elements of the traditional service, which has retained a similar structure for more than 1,000 years, while also recognising the spirit of our times.
Charles's coronation is expected to be on a smaller scale and shorter, with suggestions that it could last just one hour rather than more than three.
It is expected to be more inclusive of multi-faith Britain than past coronations, but it will be an Anglican service.
Guest numbers will be reduced from 8,000 to around 2,000, with peers expected to wear suits and dresses instead of ceremonial robes, and a number of rituals, such as the presentation of gold ingots, will be axed.
The Queen Consort is to be crowned alongside Charles during the ceremony.