Internationally acclaimed violinist Min-Jin Kym, 32, was on her way to catch a train when her Stradivarius, which is more than 300 years old, was stolen by a gang of three opportunist thieves targeting passengers at Euston station in London.
The instrument was in a black case which also contained two valuable bows.
One of only 450 in the world, it will prove difficult to sell as dealers would immediately recognise its unique label and markings.
Insurers have offered a £15,000 reward for information.
The South Korean-born violinist had stopped at Euston for a sandwich and a coffee with a friend at a branch of Pret a Manger outside the station before travelling to Manchester to visit family and friends.
She placed her violin on the ground for only a short time before it vanished. Hafid Salah, who was working in Pret at the time, said: ‘She and her friend were on computers and iPhones and not looking at their bags.
‘She came up to me at the counter and said, “Have you seen my bag? Call the police. Can you get the CCTV? You have to do something.” She was really upset and panicking.’
Detectives believe an opportunist gang grabbed the bag without having any idea of its value.
The instrument, worth at least £1.2million, has identifying repair marks under its bridge. It was on long-term loan to Miss Kym but insurers refused to comment on who owns it.
Yesterday Miss Kym was too distraught to speak about the theft, which took place around 8.30pm last Monday.
She began playing the violin at the age of six and made her international debut with the Berlin Symphony Orchestra when she was 13.
She went on to study with the violin virtuoso Ruggiero Ricci who said she was ‘the most talented violinist I have ever worked with’.
Sarah Ottley, of the Lark Insurance Broking Group, which is offering the reward, said: ‘Instruments like this are easily recognisable by dealers or repairers.
‘We would urge anyone who might be able to help us to contact the police or Crimestoppers immediately.’