The number of people visiting Blenheim Palace, Woodstock, is on the slide despite a record 71 million tourists flocking to England's other historic properties.
Chief administrator Paul Duffie said new attractions around the country - particularly theme parks - had hit traditional attractions such as Blenheim Palace.
He said: "Historic houses are having to diversify by staging events. We have the horse trials in September and the craft fairs in May."
Mr Duffie hopes better summer weather will boost the numbers of visitors to the palace.
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