Written by Sara Spary, CNN

A rare coin featuring Britain's King Edward VIII, who abdicated to marry American divorcee Wallis Simpson , has sold for a record £1 million ($1.3 million).

The Royal Mint, which makes all coins for the United Kingdom, said it had sold the historically significant coin to a private collector.

The reverse side shows "1937," the intended date of release. Credit: The Royal Mint

The Edward VIII Sovereign, as the coin is known, is one of the most coveted coins in the world and was produced as part of a "trial set" for the King when he ascended the throne in January 1936.

However, they were never circulated to the public because the King abdicated that same year to marry Wallis Simpson.

As King and head of the Church of England, the monarch was forbidden to marry a divorcee.

In his abdication speech, he famously said: "I have found it impossible to carry the heavy burden of responsibility and to discharge my duties as King as I would wish to do without the help and support of the woman I love."

The King further broke with tradition with the design of the coin. Historically, it is the custom that each monarch should face the opposite way to his or her predecessor when new currency is produced.

However, Edward insisted on being depicted facing the same way as his father, King George V, because he preferred his left profile.

Just six coins were made as part of the trial set, the Royal Mint said, and the coin is one of only two in private ownership. The remaining four are in museums.

It was previously owned by a collector in the United States, who bought it for £516,000 (around $674,000) at an auction in 2014.

The King preferred his left side. Credit: The Royal Mint

The £1 million sale price means it is the most expensive British coin ever sold.

"The Edward VIII Sovereign is one of the rarest and most collectible coins in the world, so it's no surprise that it has set a new record for British coinage," Rebecca Morgan, head of collector services at The Royal Mint, said.

"We were delighted to be able to locate such a special coin for our customer, and bring it back to the UK to make history once more."

Matt Curtis, from Royal Mint collector services, added: "This Sovereign is significant not only because of its rarity, but because it sits at the heart of an international story and has been treasured by collectors in both the UK and US.

"The Edward VIII Sovereign is certainly one of the rarest coins in the world but we hope it will help inspire others to look through their homes and see what treasures are hidden."