The British Museum is to lend Iran one of its most famous antiquities, which is regarded as the first charter of human rights, 30 years after its loan to the Imperial regime triggered a fierce diplomatic row.

The inscriptions on the clay drum known as the Cyrus Cylinder detail the conquest of the Babylon of Belshazzar and Nebuchadnezzar by the 6th-century BC Iranian King of Kings, Cyrus the Great. 

The victory made Cyrus the Great the leader of the first world empire, stretching from Egypt to China. Cyrus proved a model ruler. He describes on the cylinder measures of relief for the inhabitants of Babylon and the return to their homelands of people held by the former kings, thought to have included the Jews.

The cylinder, which would have been used as a foundation stone to a building, was found in Babylon, in modern Iraq, by a British Museum dig in the 19th century.

It has only left once since, for the loan in 1971. Its return visit to the National Museum of Tehran in 2006 will follow a generous loan by the Iranians, who are to send 50 antiquities for the British Museum's exhibition on the splendors of Ancient Persia, planned for September next year.

In the past 30 years relations between British and foreign museums have been transformed, with loans between countries now commonplace. But research by The Art Newspaper has shown that in the very different climate of 1971, the loan prompted a furious row.

The Shah had expressed his desire to "borrow" the cylinder through the British ambassador, but the suggestion was rejected by the "Foreign Office". Officials were furious when they found that the British Museum, with the agreement of its trustees, had gone ahead anyway.

One official, in recently released papers from the National Archives, said the museum should not act this way to "countries with ultra-nationalistic ambitions". Another said: "If the [British] Museum find they have dug a pit for themselves, it will be for them to climb out."

The Shah made the cylinder the star exhibit in a museum set up to mark the 2,500th anniversary of Cyrus's establishment of the Iranian monarchy in Persepolis. Facing criticism from Western countries for creating an uneasy situation for them. The celebration was a blow to the western image, since Iranians used it to prove that Iran is the birthplace of human rights.

However, the British ambassador later suggested it should be presented [returned] to Iran to gain diplomatic and military co-operation from the imperial government. The museum refused under the advice of Foreign Office. Return of the Cyrus cylinder to Iran, could have been a beginning of return of other civilizations' stolen relics to their homelands, such as Rosetta Stone and Elgin Marble.