His grandfather, King George VI, used the Tudor crown design during his 15 years on the throne, as did Edward VIII briefly before him, George V and Edward VII.

St Edward’s Crown is depicted on royal insignia across the Commonwealth, but in the UK is most commonly associated with Royal Mail.

Since the 1800s, each post box has carried a Royal Cypher to show which monarch was on the throne when it was constructed.

While more than half now carry the Queen’s EIIR crest, some 15 per cent still display the mark of George V, who reigned from 1910 to 1936, while others are said to date back to Edward VII and even Queen Victoria’s reign.

When Prince Charles becomes king, the introduction of the new cypher on insignia such as buttons, liveries and uniforms will be gradual, but fixed installations such as post boxes will not change.

St Edward’s Crown, which is kept in the Tower of London’s Jewel House, is reserved only for the act of coronation.

With two dipped arches topped with a jewelled monde and a cross pattee, it is studded with 444 precious and semi-precious stones.