The gold rosary beads carried by Mary Queen of Scots to her execution in 1587 have been stolen.
The items – described as of “great historical importance” – were taken during a raid at Arundel Castle, West Sussex.
Gold and silver treasures worth more than £1 million were stolen.
Police said: “The rosary is of little intrinsic value as metal, but as a piece of the Howard family history and the nation’s heritage it is irreplaceable.
“The items were taken by force from a display cabinet along the public route.”
A 4×4 saloon car perhaps connected to the robbery was found burnt out in nearby Barlavington.
A Sussex Police spokesman said:
“At 10.30pm on Friday 21 May, castle staff were alerted of a break in after a burglar alarm had sounded.
“Police were on the scene within minutes.
“Various items have been stolen of great historical significance.”
A spokesman for Arundel Castle Trustees said:
“The stolen items have significant monetary value, but as unique artefacts of the Duke of Norfolk’s collection have immeasurably greater and priceless historical importance.
“We therefore urge anyone with information to come forward to the police to assist them in returning these treasures back where they belong.”
Detective Constable Molly O’Malley of Chichester CID said:
“If you were in Arundel on Friday evening and saw any suspicious activity around the area of the castle, please contact us either online or by calling 101, quoting Operation Deuce.
“In addition, the castle only re-opened to visitors on Tuesday May 18th so if you were visiting during the past few days do you on reflection recall anyone behaving at all suspiciously?
“If you are offered or hear of anyone offering for sale any of the items stolen, we would also like to hear from you.”