Lord and Lady March were assaulted and tied up during a raid in which more than £700,000 worth of heirlooms were stolen from Goodwood House, police have revealed.
Detectives have appealed for more information about the the break-in at the stately home in West Sussex at around 4.30am on Wednesday.
Detective Inspector Till Sanderson said: "This was a frightening ordeal and I pay tribute to the courage of Lord and Lady March.
"The intruder broke in after scaling a ladder to an upstairs window and his activity disturbed Lady March, who went to investigate.
"She disturbed the man, who was alone, and he pushed her and struck Lord March on the head, causing an injury to his ear.
"Lady March was then forced to open a safe and the man helped himself to jewellery.
"The couple were bound before the robber escaped with the items."