A rare 1820 pattern £5 proof coin is expected to make £200,000-250,000 ($313,812-392,265) at Dix Noonan Webb.
The piece was commissioned while George III lay on his deathbed.
The king died before the dies could be completed, meaning that it could not be released into circulation.
Instead around 25 were struck as commemorative pieces. Of these, six were given to various museums and institutions.
The other 19 were distributed among various collectors and the king's own family. It remains one of the rarest British coins in history and as such is highly sought after among collectors.
Christopher Webb, head of coins at Dix Noonan Webb, comments: "This coin is something very special.
"It is a wonderful example of the coin engraver's art. But it also marks the end of the reign of George III which had seen the loss of the American colonies, the French Revolution, the rise and fall of Napoleon and the birth of the Industrial Revolution. Here beauty combines with history."
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