A suspect has been arrested over the theft of rosary beads that belonged to Mary Queen of Scots.
Treasures worth more than £1million were taken in the raid on Arundel Castle despite police arriving within minutes of the alarm going off.
Other missing items include gold and silverware and a series of coronation cups.
But the rosary beads, which Mary carried with her to her execution in 1587, were the pride of the collection at the West Sussex castle.
Police carried out eight raids on addresses in Gloucestershire, Oxfordshire and in Eckington, Worcestershire, where the 45-year-old suspect was seized.
Officers did not say whether any of the stolen treasures had been recovered.
'Our investigation into the Arundel Castle burglary remains live,' said Detective Inspector Alan Pack of Sussex Police. The break-in was on May 21.
A man, 45, has been arrested over the theft of rosary beads that Mary Queen of Scots carried with her to her execution in 1587, were the pride of the collection at the West Sussex castle
Detective Inspector Pack added: 'This action marks a significant step in our enquiries.
'I would encourage anyone with further information about this burglary to contact us and also remind people that the insurers have offered a substantial reward should any of the property be recovered intact.'
The collection, valued at more than £1 million, comprised Mary Queen of Scots' Rosary Beads, seven gold and silver-gilt coronation cups (George II, George III, George IV, William IV, George V, George VI and Elizabeth II), gold Earl Marshal's baton, gold and enamel baton, other miscellaneous items including 10 silver-gilt Apostle Spoons, a silver-gilt casket with hunting and fishing scenes, and a silver-gilt mug.
Police carried out eight raids on addresses in Gloucestershire, Oxfordshire and in Eckington, Worcestershire, where the 45-year-old suspect was seized. They have not said if the rosary beads have been recovered
A Sussex Police spokesman at the time 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.'
Staff were alerted to the break-in at 10.30pm on May 21 after a burglar alarm went off and police were scrambled to the scene.
A 4x4 saloon car was later found burnt out and abandoned.
A spokesman for Arundel Castle Trustees said at the time: '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.'