Iran has severed all cultural ties with the British Museum in protest at the delay of the loan of an ancient Persian treasure, it emerged yesterday.

British Museum officials were due to lend the 2,500-year-old artefact to Iran's national museum last month, but announced they were holding on to it to do some more research.

The clay cylinder - which was acquired by the museum after being discovered in 1879 - is regarded as the world's first declaration of rights.

Hamid Baghaei, head of Iran's Cultural Heritage and Tourism Organisation, said the decision to keep the cylinder was unacceptable and politically motivated.

He told Reuters news agency: "The Cultural Heritage Organisa-tion has cut all its relations and co-operation with the British Museum."

Mr Baghaei has reportedly said his organisation will send a letter of complaint to the United Nations' cultural body, Unesco.

He has also threatened to write to all world museums to warn them against working with the British Museum.

The British Museum said in September that it would be delaying the hand-over of the 2,500-year-old clay cylinder due to unspecified "practicalities".

But last month curators revealed they had discovered two pieces of cuneiform tablet that shed more light on the Cyrus Cylinder, and the new texts needed to be properly studied.

They proposed an international workshop at the British Museum in June, to study and assess the new pieces

The Cyrus Cylinder was written in Babylonian cuneiform on the orders of the Persian king Cyrus the Great after his conquest of Babylon in 539 BC.

The cultural row comes amid heightened tension between the UK and Iran as a result of the oil state's nuclear activities.