Thieves have used power tools to smash an armoured glass case and steal a diamond-encrusted tiara "worth millions".
The Portland Tiara was stolen in a heist on the Welbeck Estate in Worksop, Nottinghamshire.
It was created for Winifred, Duchess of Portland, to wear to the 1902 coronation of King Edward VII.
An expert said he feared it could be broken up and the diamonds sold off separately.
Police said they believed a silver Audi S5 found burnt out in Cross Lane, Blidworth, had been used by the thieves.
The tiara is centred on the Portland Diamond, which is flanked by two diamond drops and other pendant diamonds, all set in gold and silver.
The burglars broke into the Portland Collection Gallery between 21:45 and 22:00 GMT on Tuesday and snatched the object as alarms sounded.
"Security services missed the thieves by 90 seconds and the police arrived on the scene two minutes later," said a spokesperson for the Welbeck Estate.
Nottinghamshire Police said it was following "a number of lines of inquiry" but attention was focused on the recovered car.
Richard Edgcumbe, curator of jewellery at the Victoria and Albert Museum in London, described the jewelled headpiece as "one of the great historic tiaras of Great Britain".
James Lewis from Bamford auctioneers in Derby said it had been "designed in a period when money was no object".
Mr Lewis said: "It could be worth millions - but it is such a well-known piece it can't come up for public sale.
"The most horrendous thought is it could be something called a 'breaker', where the diamonds are taken out and sold individually."
A diamond brooch made from stones removed when the tiara was remodelled was also stolen in the raid.
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Welbeck Abbey has been home to the Dukes of Portland since 1607.
The Portland Collection, housed in the estate's converted gas works, is made up of paintings, jewellery and other art collected by the Dukes over 400 years.