Lord and Lady March have been assaulted and tied up by a burglar who stole more than £700,000 of family heirlooms.
The break-in at Goodwood House in West Sussex took place at about 04:15 GMT on Wednesday, Sussex Police said.
The intruder used a ladder to get into the house through an upstairs window before being disturbed by 54-year-old Lady March.
The man pushed her, and hit Lord March, 61, on the head before tying the pair up and taking the jewellery.
Staff at the 17th Century home, near Chichester, found the couple still tied up when they arrived for work at 06:30.
A 26-year-old man from Hampshire has been arrested in connection with the raid and released on bail until February.
A diamond tiara and a ring given by Charles II to his mistress were among more than 40 items stolen from Goodwood House.
Sussex Police said the stolen tiara, dating from 1820, was worth in the region of £400,000.
Lord March, who founded the Goodwood Festival of Speed in 1993, and his wife, also known as Jane Astor, were "devastated" at the theft of "priceless" items, police said.
Gems expert Filippo Guerrini-Maraldi, of insurance broker RK Harrison. said the jewellery was likely to be broken up and sold on.
"The chances are they will dismantle the tiara to take the diamonds," Mr Guerrini-Maraldi said.
"The metal will be melted down and used for another item."
Mr Guerrini-Maraldi said the thieves may not have been looking for specific items.
"They might be main-chancers and if they stumble across a tiara such as this they have hit the jackpot," he said.
Ch Insp Justin Burtenshaw, of Sussex Police, said: "They [Lord and Lady March] are devastated at the loss of these priceless items.
"I would like to praise the courage of both of them, both during the ordeal and afterwards now as we investigate."
Jewellery stolen from Goodwood House
- 1820 diamond tiara worth £400,000
- Diamond necklace from the first half of the 19th Century worth £200,000
- Emerald and diamond ring engraved with Duchess's coronet and monogram CL for Louise de Keroualle, Duchess of Portsmouth, the mistress of Charles II
- Antique Rolex and Girard Perregaux watches