A "nationally significant" bronze medieval jug has been stolen from a Bedfordshire museum.
The Wenlok Jug was taken from the Stockwood Discovery Centre in Luton at about 23:00 BST on Saturday.
In 2005 it was nearly sold abroad, but a temporary export ban provided the opportunity for Luton Museum to raise the £750,000 needed to buy it.
Director of Museums, Karen Perkins, called the theft "extremely serious and upsetting".
She said: "We are extremely proud that the Wenlok Jug is part of the collections at Stockwood Discovery Centre and are working extremely closely with police and investigators to do all we can to recover it.
"The Wenlok Jug is a nationally significant medieval object."
The jug is a very rare example of metalwork that can be associated with royalty from the 1400s.
It is decorated with coats of arms and badges and is inscribed with the words "My Lord Wenlok".
In May 2005 it went up for sale at Sotheby's and was nearly sold to New York's Metropolitan Museum.
However its export was stopped in October of that year by culture minister David Lammy after experts ruled it was of "outstanding significance" for the study of bronze-working in medieval England.
It is thought the jug was made for either William Wenlock, who died in 1391 and was canon of St Paul's Cathedral, or his great-nephew John, the first Lord Wenlock, who was a major figure in the 15th century.