Sussex Police are appealing for information after £700,000 of family heirlooms were stolen from Goodwood House last week (13 January).
The intruder broke into the house at 4.30am after scaling a ladder to an upstairs window.
The man was disturbed by Lady March who had gone to investigate his activities.
The intruder pushed her and struck Lord March, 61, on the head before forcing Lady March to open the safe.
The burglar bound and gagged the couple before escaping with more than 40 items including emerald, diamond and sapphire rings, earrings, bracelets and antique Rolex and Girard Perregaux watches.
Among the most treasured items stolen were a £400,000 diamond tiara, an antique diamond necklace worth £200,000 and an emerald intaglio and diamond ring given by Charles II to his mistress Louise de Keroualle.
A member of staff called the police at 6.30am when he arrived at work.
“This was a frightening ordeal and I pay tribute to the courage of Lord and Lady March,” said detective inspector Till Sanderson. “They are devastated at the loss of these treasured items, many of which are irreplaceable.”
A nationwide appeal has been launched to trace the stolen jewellery and forensic teams are working with detectives to investigate the circumstances of the theft.
“We are doing all we can to try and trace the stolen treasures, liaising with auction houses, stately homes, dealers and those with specialist knowledge,” said Inspector Sanderson.
A 26-year-old man from Hampshire has been arrested on suspicion of robbery and has been bailed until February while investigations continue.
The Goodwood Estate is home to the Glorious Goodwood race meeting. The 2016 racing season opens on 30 April. The seven-race card meeting has an artisan food festival running alongside.