Thieves made off with a £1.2 million antique Stradivarius violin when a classical musician went to buy a sandwich.
Min-Jin Kym, 32, was targeted when she sat down to eat at a branch of Pret A Manger outside London's Euston railway station.
The Korean-born violinist suddenly noticed the black case containing the 300-year-old instrument was missing.
Police today appealed to the public to help them track down the instrument which they believe would be almost impossible to resell.
Detective Inspector Andy Rose, of British Transport Police (BTP), said a £15,000 reward had been put up by Miss Kym's insurers.
He said: "These items hold enormous sentimental and professional value for the victim, but although they are extremely valuable, it would be very difficult to sell them on as they are so rare and distinctive that they will be easily recognised as stolen property.
"It's possible the instrument will be offered for sale within the antique or musical trade and we ask anyone who has any knowledge of the violin's whereabouts to come forward so it can be returned to its rightful owner."
The stolen instrument was a 1696 Antonio Stradivarius violin, valued at about £1.2 million. A £62,000 Peccatte bow was also in the case.
The violin has a number of unique identifying marks, including visible repair marks under the instrument's bridge and a specially-moulded chin strap.
The Met's Arts and Antiques Unit has been alerted and the items registered on the London Stolen Arts Database.
Detectives believe the items could have been taken to another major city, possibly Liverpool or Newcastle.
Sarah Ottley, of Lark Insurance Broking Group, said: "Instruments like this are almost impossible to replace as they are so unique.
"However, this does mean they are easily recognisable by dealers or repairers.
"We would urge anyone who might be able to help us reunite the violin and bows with their owner to contact the police or Crimestoppers immediately."
The theft took place between 8.30pm and 9pm on Monday November 29.
* Anyone with information should call BTP on 0800 40 50 40 or Crimestoppers, anonymously, on 0800 555 111.