An export ban has been placed on a rare 14th Century scientific instrument uncovered during building work in Kent to stop it leaving the country.
An anonymous telephone bidder paid £138,000 for the item in March
The astrolabe quadrant - dubbed the "pocket calculator" of its age - sold for £138,000 at auction in March.
Culture Minister Margaret Hodge postponed plans to export the item until February 2008 to allow money to be raised to keep it in the UK.
The item is said to be one of only eight examples in the world.
Ms Hodge's ruling followed a recommendation by the Reviewing Committee on the Export of Works of Art and Objects of Cultural Interest.
Committee member, Catherine Johns, said: "This small scientific instrument, with its many unique details, provides us with a vivid insight into the sophisticated mathematical and astronomical knowledge of the 14th century, the age of Chaucer."
The Department for Culture, Media and Sport (DCMS) said the export ban might be extended to June 2008 if a serious intention to raise the recommended price of £350,000 was made.
An anonymous telephone bidder paid £115,000 at Bonhams in London, which rose to £138,000 with auction commission and tax.
The brass instrument was found during excavations for an extension to a restaurant.
The site of the find is known as the House of Agnes, a 17th Century inn on the main road out of Canterbury to London.