Thieves have delivered a fresh blow to a monument on the Castle Howard Estate which is listed in English Heritage's Register of Buildings at Risk.
Flagstones measuring 2ft by 3ft and valued in total at £3,000 have been stolen from around the base of the Monument to the Seventh Earl of Carlisle.
A spokesman at Malton police station said Castle Howard Estates had reported the theft on Tuesday, and said it had occurred some time over the last 12 months.
He urged anyone with information about the offence to telephone Crimestoppers on 0800 555111 and leave details for PC 403 Steve Leach.
Last June, English Heritage published its second Register Of Buildings At Risk.
It contained details of 176 Grade I and II* Listed Buildings in the Yorkshire and Humber region - 76 of them in North Yorkshire.
They included the Monument to the Seventh Earl of Carlisle, located in Main Lane, Bulmer, which English Heritage described as being in "poor" shape.
English Heritage stresses that inclusion in the at risk register does not imply any criticism of property owners, and says many of them are actively seeking ways to secure their future.
Duncan Peake, agent for the Castle Howard Estate, said: "We are deeply saddened by this act of wanton theft and vandalism to this historic monument.
"We are, of course, concerned by the cost of replacing the stolen flagstones, but our most immediate task is to prevent any further deterioration of the monument caused by the theft, as further damage may occur if urgent action is not taken.
"To this end, we are liaising closely with English Heritage to determine the best course of action."
He added: "Sadly, this problem is one faced by many owners of historic monuments and significantly adds to the cost of preserving our nation's heritage."
Mr Peake said the stolen flagstones varied in size, but most measured approximately 3ft by 2ft and could be identified as they had a fine tooling finish on one side.
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