The priceless heirloom - made by Cartier in 1902 - was pinched along with a diamond brooch from the Portland Collection Gallery
Police are hunting a gang of thieves who smashed into an armoured glass display case at a historic estate before fleeing with a priceless diamond tiara.
The famous Portland Tiara - seen by countless members of the public and described as a 'national treasure' - was pinched from The Welbeck Estate in Worksop, Nottinghamshire on Tuesday night.
Burglars broke into the Portland Collection Gallery on the estate sometime between 9.45pm and 10pm and made off with the heirloom.
They also stole a diamond brooch from the same display case while the alarms were sounding.
Richard Edgcumbe, Curator of Jewellery at the Victoria and Albert Museum in London, said: "The Portland Tiara is one of the great historic tiaras of Great Britain.
"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."
Police are appealing for information about a silver Audi S5 suspected to have been involved in the offence.
Detective Inspector Neil Humphris 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 Estate and Cross Lane in Blidworth between 9.45pm and 10.30pm on Tuesday night?
"If you have any information about this vehicle or the offence itself 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 dash cam footage of this vehicle."
The centre-piece of the tiara is the Portland Diamond, which dates from the 19th century and is flanked by two diamond drops and other pendant diamonds, all set in gold and silver.
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.
Meanwhile, the brooch is composed of diamond clusters that previously stood at the apex of the tiara.
These gems can be seen on the tiara in a painting of Duchess Winifred at the anointing of Queen Alexandra but are absent from it in a 1925 portrait of the Duchess, in which she wears the tiara low on her head as a bandeau.