Iran has severed all cultural ties with the British Museum over the institution’s failure to hand over an ancient Persian treasure.

At the centre of the diplomatic row is a 2,500-year-old cuneiform tablet, known as the Cyrus Cylinder, which most historians regard as the world’s first declaration of human rights.

Curators had been due to lend the artefact to Tehran last month, but announced that the handover would be delayed after the discovery of new tablets that they believe could help its research. The delay has provoked the anger of Iranian officials, who announced an end to dialogue yesterday in protest at a decision that they believe is politically motivated.

Hamid Baghaei, head of Iran’s Cultural Heritage and Tourism Organisation, said that the move to keep the cylinder was unacceptable. “The Cultural Heritage Organisation has cut all its relations and co-operation with the British Museum,” he said.

Mr Baghaei added that his organisation would send a letter of complaint to Unesco, the United Nations’ cultural body, and threatened to write to all international museums warning them against working with the British Museum.

The museum expressed “great surprise” at Iran’s reaction, which now threatens to end a symbolic friendship that has survived despite decades of political turmoil.

The museum said in a statement last night that it had confirmed its intention to lend the artefact and associated fragments of clay tablet to the National Museum of Tehran in the second half of July in a phone call to Iranian officials last week. This was followed with an e-mail and faxed letter to Mr Baghaei a few days later.

“The new announcement from Mr Baghaei therefore comes as a great surprise. The British Museum has acted throughout in good faith, and values highly its hitherto good relations with Iran. It is to be hoped that this matter can be resolved as soon as possible.”

The statement added: “The British Museum has a longstanding policy of lending its unparalleled collection as widely as possible across the world. This cultural exchange is a vital part of the museum’s commitment to being a museum for the world ... allowing valuable dialogues to develop independently of political considerations.”

The cultural row comes amid heightened tension between Britain and Iran over its nuclear activities.

When Neil MacGregor, director of the British Museum, and Mr Baghaei signed the loan agreement in January 2009, six months before the violence surrounding the presidential elections, their friendly encounter was hailed as a diplomatic breakthrough. At the time even the British Ambassador in Tehran was struggling to maintain a dialogue.

The clay cylinder, which was acquired by the museum after its discovery in 1879, was written in Babylonian cuneiform on the orders of the Persian king Cyrus the Great after his conquest of Babylon in 539BC. It remains of huge significance in Iranian, Iraqi and Jewish history.

The Iranian human rights lawyer Shirin Ebadi, accepting her Nobel Peace Prize in 2003, hailed the charter as “one of the most important documents that should be studied in the history of human rights”, and cited Cyrus as a leader who “guaranteed freedoms for all”. Last week she reiterated her support for lending it to Tehran. “Whatever happens inside Iran has nothing to do with sending the cylinder to Iran,” she said.