The amount Maxim Viktorov paid for the rare violin by 18th-century master craftsman Giuseppe Guarneri, known as Giuseppe del Gesu, wasn't revealed by Sotheby's, which brokered the sale.
However, the Times of London said the price is believed to have been "well in excess" of the world auction record of $3.54 million, set when a Stradivarius was bought at Christie's New York in 2006.
The newspaper said Viktorov, who is also a musician, owns an extensive collection of violins and frequently lends them out to performers.
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