A new 50p coin will enter circulation from Monday.
Some 1.3 million coins are being made available on "limited release" at UK post offices from February 7.
The new coin will mark the Queen's Platinum Jubilee.
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As many as five million of the coins could be released, as the Queen celebrates a staggering 70 years on the throne.
It is the first time a 50p will ever be used to mark a royal event and the coin will feature a clean reverse design that comprises the number 70.
In 2012, a £5 coin was released to commemorate the Queen's Diamond Jubilee.
Post Offices in Windsor and close to Sandringham were among the first branches to receive the new coin.
The coin can be used as any other 50p coin, though people may wish to hang on to it as a keepsake.
All other 50p coins will remain valid currency in the UK.
Nick Read, chief executive at the Post Office, said: "We are thrilled to be part of the celebrations for Her Majesty's Platinum Jubilee year and that our 11,500 Post Offices across the whole of the United Kingdom will exclusively start the circulation of this special 50p coin today.
"Postmasters are at the heart of their local communities and have been throughout our 360-year history. I have no doubt that this will be a real honour and privilege for Postmasters and their teams."
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