A temporary export ban has been placed on a sapphire and diamond coronet that belonged to Queen Victoria, preventing it from being sold abroad.
The coronet, designed by Prince Albert for their wedding in 1840, is at risk of being exported unless a UK buyer matches the £5m asking price.
The temporary ban was imposed after the owner applied for an export licence.
Culture minister Matt Hancock, who imposed the ban, said it symbolised one of the UK's "most famous love stories".
The 11.5cm (4.5in) wide coronet is mounted with 11 sapphires, which are all set in gold, with diamonds set in silver.
Experts consider it to be one of the most important jewels of Queen Victoria's reign, matching a sapphire and diamond brooch given to her by Albert the day before their wedding.
Following Albert's death in 1861, Queen Victoria refused to attend the State Opening of Parliament until 1866, when she wore the coronet.
Both the coronet and brooch also featured in one of the most famous official portraits of the young Queen Victoria, in 1842, by Franz Xaver Winterhalter.
The coronet was given by King George V and Queen Mary to Princess Mary on her marriage to Viscount Lascelles in 1922. It was later sold to a dealer in London, who then sold it to the export licence applicant.
The temporary ban followed a recommendation by the reviewing committee on the export of works of art and objects of cultural interest, which is administered by the Arts Council.
It recommended the restriction on the grounds of the coronet's "close connection with our history and national life, and its outstanding significance for the study of the young Queen Victoria".
Committee member Philippa Glanville described the piece as "exquisite", adding: "It evokes vividly the shared romantic taste of the time, and its form has become familiar through many reproductions.
"Its departure would be a great loss, given its beauty, its associations and its history."
Mr Hancock said it was "one of the most iconic jewels from a pivotal period in our history".
"I hope that we are able to keep the coronet in the UK and on display for the public to enjoy for years to come."
The Department for Culture Media and Sport said a final decision over the export licence will be deferred until 27 December.