The exhibition, which brings together treasures from the National Museum of Iran, the Louvre and the British Museum itself, aims to tell the story of an empire that, between 550BC and 330BC, was the biggest the world had seen .
Highlights include the "Cyrus Cylinder", a barrel-shaped inscribed foundation deposit sometimes referred to as the first declaration of human rights because of its reference to religious tolerance.
This, say organisers, goes some way to exploding the myth of Persia as a ruthless and despotic war machine.
Other key exhibits include a large stone dog that guarded the palace at Persepolis which was burnt by Alexander. Casts of reliefs from Persepolis, taken by a British expedition in 1892, also go on show for the first time. Other items relate to the three great rulers of the region, Cyrus, Darius and Xerxes.
John Curtis, keeper of the museum's Ancient Near East department said: "We call it a forgotten empire not because it's been forgotten by the Islamic Republic of Iran or any other country in the Middle East but because it's been forgotten in western Europe."
Forgotten Empire: the World of Ancient Persia, sponsored by BP, runs from 9 September to 8 January; admission, £8.Reuse content