The historic round pillar box, considered a British classic, is on the High Street in Great Cambourne, south Cambridgeshire.
Helping unveil the new addition to the village, youngsters from Vine Inter-Church Primary School in Upper Cambourne gathered at the postbox and posted the first letters into the freestanding stainless steel structure – handwritten notes addressed to the King.
Charles has granted the Royal Mail special use of his monogram, which is his personal property.
His cypher consists of his initial C, with the Roman numeral III and an R for Rex – Latin for King – under a Tudor Crown.
Royal Mail unveil first post box with King’s cypher
The unveiling continues the 170-year-old tradition of stamping red postboxes with a royal cypher, dating back to the reign of Queen Victoria.
Emma Gilthorpe, Royal Mail’s chief executive, said: “More than 115,000 postboxes across the UK have recorded the succession of monarchs since the first box bore the cypher of Queen Victoria.
“We thank His Majesty for granting us the use of his personal cypher as we begin the next chapter in the story of the iconic Great British postbox in Great Cambourne.”
A commemorative plaque records the special status of the historic postbox.
A number of new or replacement postboxes have been installed since Charles acceded to the throne in 2022, but existing stocks carrying the cypher of Queen Elizabeth II were used until now, in order to prevent wastage.
Existing boxes will be kept as they are, retaining the cypher from the period in which they were installed.
The King’s cypher will also be applied to the doors of the Royal Mail’s fleet of vehicles, replacing the late Queen’s, with an initial roll-out of 100 trucks on Friday, followed by a further 290 over the next six weeks.
In total 3,636 vehicles will be updated between August and October this year.
Coinciding with the unveiling, the Royal Mail announced a change in its brand logo, known as the Cruciform.
The logo with its yellow double lined lettering of Royal Mail, used on the company’s vehicle livery, buildings, website and stationery, will now feature the rounded Tudor Crown used by the King, rather than the arched St Edward’s crown used by the late Queen for her cypher.
But, in line with guidance from the royal household and in keeping with the King’s eco-credentials, it will only be applied to new or replacement signage, stationery and vehicles to avoid unnecessary costs and waste.
The King’s cypher will, however, not be applied to new postboxes in Scotland.
Since 1955 Scottish postboxes have been distinguished by the St Andrews crown rather than the monarch’s cypher.
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At the start of the late Queen’s reign in 1952, there were objections in Scotland to her being styled Elizabeth II because the Tudor queen Elizabeth I was never a queen of Scotland.
A pillar box in Edinburgh bearing the EIIR cypher was defaced and later blown up.
Its replacement was left blank, and the UK Government later agreed only the Crown of St Andrew would be used for postboxes and Royal Mail vehicles in Scotland.
There will be no change of policy on future postboxes in Scotland, Royal Mail said.