The famous Portland Tiara - a national treasure which has been seen by countless members of the public - has been stolen from the Portland Collection Gallery in Worksop, police have revealed.
Burglars broke into the gallery on the Welbeck Estate between 9.45pm and 10pm on Tuesday.
They stole the tiara and a diamond brooch from an armoured glass display case while the alarms were sounding.
"The Portland Tiara is one of the great historic tiaras of Great Britain," said Richard Edgcumbe, curator of jewellery at the Victoria and Albert Museum in London.
"Since its creation by Cartier in 1902, using diamonds from the historic collections of the Dukes of Portland, it has been recognised as a jewel of supreme importance, a superb design magnificently executed."
The 6th Duke of Portland commissioned Cartier to create the Portland Tiara for his wife, Winifred, Duchess of Portland.
She wore it to the 1902 coronation of King Edward VII.
The centre-piece of the tiara is the Portland Diamond, which dates from the 19th century.
It is flanked by two diamond drops and other pendant diamonds, all set in gold and silver.
James Lewis, from Bamford auctioneers in Derby, told the BBC the tiara 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 spokesperson for the Welbeck Estate said: "The tiara is part of the historic Portland Collection.
"It has been on display to the public at Welbeck since March 2016 and was previously loaned to the V&A Museum for its Tiara exhibition in 2002."
Police apeal about silver Audi
The police investigation continues and detectives are appealing for information about a silver Audi S5 suspected to have been involved in the offence.
Detective Inspector Neil Humphris, of Nottinghamshire Police, said: "We're pursuing a number of lines of enquiry but we believe there are people out there who may have crucial information that could help with our investigation.
"We particularly want to hear from anyone who has any information about a silver Audi S5 which is suspected to have been involved in this offence. This vehicle was found abandoned and burnt out in Cross Lane, Blidworth, about half-an-hour after the incident.
"Did you see this vehicle in the Welbeck area around the time of the burglary or anywhere between the Welbeck Estates and Cross Lane in Blidworth between 9.45pm and 10.30pm on Tuesday?
"If you have any information about this vehicle, we'd urge you to please come forward and contact us as soon as possible. We'd also like to hear from any drivers who may have dashcam footage of this vehicle.
"We're asking people to please call us on 101, quoting incident number 856 of November 20, or Crimestoppers anonymously on 0800 555 111."