The Magnus Princeps bronze portrait medal of Sultan Mehmed II, c.1460, which was only discovered in late 2000. Estimated at £300,000-400,000, it provides the earliest known portrait of one of history’s greatest military commanders.

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The sale comprises 150 of the rarest and highest quality Islamic coins from the first to the 13th centuries of the Hijri and includes some of the finest examples of the coinage of Mecca ever to be offered at auction.

"Although the auction contains items from more than one vendor, the heart of the sale comprises a collection begun in the 1950s by a private collector," said a spokesman for Baldwin's.

Highlights include the rarest of all Islamic coins, the Umayyad dinar, the earliest gold coin which can be said with certainty to have come from the Arabian Peninsula.

"A coin of this type and of this quality was sold at auction in London in 2011 and achieved a staggering £3.7m, making it the second most expensive coin to ever be sold at auction," said Baldwin's.

If this example matched that price, it would easily outstrip the star of the Prospero sale - acknowledged as the finest collection of Classical Greek coins to be offered at auction in more than 40 years. That was the gold stater from Pantikapaion, depicting the head of a satyr, which sold for $3.25m hammer.

Aside from coins, the Islamic sale will also offer one of the most important pieces of Islamic historical documentation. The discovery of the Magnus Princeps bronze portrait medal of Sultan Mehmed II, c.1460 in late 2000 provides the earliest known portrait of one of history's greatest military commanders.

Conceived barely 15 years after the first art medal had been produced in Renaissance Italy, the Magnus Princeps medal throws additional light on the personal interest that Mehmed II was taking in this new medium.

It carries an estimate of £300,000-400,000 for the April 25 sale in The Pine Room at The Westbury Hotel, Mayfair, London.