The Babylonian clay cylinder dates from the 6th century BC
Iran's national museum has said it will cut all ties with the British Museum in protest at a decision to delay the loan of an ancient Persian treasure.
The Cyrus Cylinder dates back to the 6th Century BC and is regarded as the world's first declaration of rights.
The British Museum says it needs to keep the artefact until the summer in order to continue its research.
But the head of Iran's state cultural organisation said the move was unacceptable and politically motivated.
"The Cultural Heritage Organisation has cut all its relations and co-operation with the British Museum," said the head of the organisation, Hamid Baqaie, according to Reuters news agency.
He said his organisation would send a letter of complaint to the UN's cultural body, Unesco, and suggested Iran would incur considerable costs because of the delay.
Symbol of early rights
He also threatened to write to all world museums to caution them against working with the British Museum.
In September, the British Museum said it would have to delay handing over the 2,500-year-old clay cylinder due to unspecified "practicalities".
But, last month, the museum said the reason it needed to keep the object was in order to compare it with two recently discovered tablets, thought to be from the same period.
The cylinder was ordered to be made by Persian king Cyrus following the conquest of Babylon.
Covered in tiny lettering called cuneiform, it is said to represent the first bill of rights and encapsulate religious tolerance.