May was the busiest month the museum has recorded. Staff members are not sure whether to credit the painting, which went on display a day before The Da Vinci Code opened at cinemas, or record rainfall. Pigment analyses of the painting La Gioconda show it was created before 1510 and that its brush strokes were probably by a left-handed painter like Leonardo da Vinci. Mona Lisa, which Leonardo worked on from 1503 to 1507, is also known as La Gioconda.

The work in Portland shows a woman who looks like the Mona Lisa subject without her smile. It is impossible to know who painted it, but its resemblance to the masterwork sent the museum searching for clues to whether it is a knock-off or a rough Mona Lisa draft. Many Mona Lisa copies exist, but they do not date from Leonardo’s time.

"There are subtle differences between the paintings, and that is not what you usually find in a copy," said the museum director, Daniel O’Leary. The museum received the painting as a bequest from a local collector. It usually keeps it in storage because it does not fit in with its 19th- and 20th-century works. But the painting was taken out of storage in 2004 to capitalise on Dan Brown’s The Da Vinci Code.