The king’s portrait will first appear on the 50p, with the reverse — or “tails” side of the coin — commemorating the life and legacy of Queen Elizabeth II.
A commemorative version of the coin was released in October, generating record visitors to the Royal Mint’s website in the 24 hours following.
A total of 4.9 million 50p coins will enter circulation across 9,452 Post Office branches throughout December.
Coins will be distributed as change when customers make purchases.
About 9.6 million 50p coins will eventually enter circulation, in line with demand.
The Post Office’s Aldwych branch in central London, near to Clarence House, is among the locations getting the new coins.
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The coins’ appearance coincides with the release of the Duke and Duchess of Sussex’s Netflix docuseries, with the royal family braced for Prince Harry and wife Meghan’s latest revelations.
“Today marks a new era for UK coinage, with the effigy of King Charles III appearing on 50ps in circulation,” said Rebecca Morgan, director of collector services at the Royal Mint.
“It’s a fantastic opportunity for coin collectors to add to their collections, or start one for the first time.
“We anticipate a new generation of coin collectors emerging, with people keeping a close eye on their change to try and spot a new 50p that bears the portrait of our new king.
“The Royal Mint has been trusted to make coins bearing the monarch’s effigy for over 1,100 years and we are proud to continue this tradition into the reign of King Charles III.”
Nick Read, chief executive of the Post Office, said: “It is a tremendous honour for the Post Office and for postmasters that the first coinage featuring King Charles III is being released into circulation via our extensive branch network.
“December is our busiest time of the year so the coin will be entering our network in a phased manner. If you don’t receive the new 50p in your change on your first visit to a post office, you may well get it in your change in a subsequent visit, so keep a look out for it.”
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The king’s effigy was created by sculptor Martin Jennings and personally approved by King Charles.
In keeping with tradition, the king’s portrait faces to the left — in the opposite direction to the late queen.
The reverse of the 50p features a design which originally appeared on the 1953 Coronation Crown.
It was struck to commemorate the queen’s coronation at Westminster Abbey and includes the four quarters of the Royal Arms depicted within a shield.
Between each shield is an emblem of the home nations: a rose, a thistle, a shamrock and a leek.
All UK coins bearing the effigy of Queen Elizabeth will remain legal tender and in active circulation.
About 27 billion coins circulating across the UK bear her portrait.
They will be replaced over time as they become damaged or worn and to meet demand for additional coins.
Historically, it has been commonplace for coins featuring the effigies of different monarchs to co-circulate, ensuring a smooth transition with minimal environmental impact and cost.