The portrait was painted in around 1560
Art experts think they may have found the world's oldest painting to feature an image of a watch.
The Science Museum is investigating the 450-year-old portrait, thought to be of Cosimo I de Medici, Duke of Florence, holding a golden timepiece.
Curators have sent their findings to renaissance experts at the Uffizi gallery in Florence, and are awaiting their comments.
The painting is being shown as part of the museum's Measuring Time gallery.
The first watches appeared shortly after 1500 in Germany and horologists believe the picture, painted by renaissance master Maso da San Friano around 1560, "may well be the oldest to show a true watch".
Coat of arms
Science Museum curator Rob Skitmore said the watch was thought to be from southern Germany.
"As Cosimo was a great patron of science and technology, it is entirely likely he would have owned a watch of this kind which he displays here with pride," he said.
"The picture shows the close linkage between science and art, especially in those days."
The painting has been in the museum's collection for 33 years after being acquired from a private donor.
As it was being taken out of storage for the gallery, curators decided to research the painting - which was when they made their discovery.
The clue to the painting's identity came when Mr Skitmore realised a seal containing the Medici coat of arms was on the back of the canvas.
He said: "In our painting Cosimo would have been about 41 and his appearance is entirely consistent with a later view of him from 1574."
The Measuring Time gallery traces the history of timekeeping and contains one of the biggest collections of clocks in Britain.