A £750,000 diamond ring, missing from the British Museum for six years, has only now been registered as lost.
Thought to have been bequeathed to the museum by an anonymous donor, the Cartier ring was reported missing to the police in 2011.
The loss was revealed with the publication of museum's annual accounts where its cost has been written off.
A spokesperson said it was museum procedure to report losses five years after discovering a missing piece.
The ring was not on public display when it went missing, the museum said.
A spokesperson said: "The ring was found missing from its proper location by British Museum staff in August 2011.
"British Museum procedure, as agreed by trustees, requires the ring formally to be reported as lost five years after the initial discovery of its absence.
"The museum has since reviewed its security and collections management procedures and dedicated significant investment to improved security across the estate."