Art of forgery: Fakes, mistakes and discoveries at the National

In 1845 when the National Gallery bought a forged painting after mistaking it for a Holbein, its director was forced to resign amid establishment uproar. Now, nearly two centuries after the blunder, the gallery's current director, Nicholas Penny, reckons self-respecting art institutions should be proud to have a few fakes in their illustrious collections.

Yesterday he announced the gallery's first ever exhibition of 40 fakes, copies and imitations in the gallery's own permanent collection, saying: "It's not a bad idea to have duds and fakes". He added: "I wish we had more fakes, I'm not worried about the reputation of the institution. It's important to know how clever forgers can be. The National Gallery is a place where we show great masterpieces but it's also a place where you can study the history of art.

"It would be very naïve for people to think it's something we should be ashamed of, or something that we should get rid of."

Dr Penny said that while he was running the National Gallery in Washington, forgeries had been "deliberately acquired". "The histories of mistakes of this kind encourages extreme caution and a certain humility," he said, adding that even expert art historians sometimes got it wrong. He cited the legendary 19th century German curator, Wilhelm Von Bode, as someone who, "made some mistakes but was one of the greatest connoisseurs alive".

The exhibition called "Close Examination: Fakes, Mistakes and Discoveries", to be staged next June, will feature the infamous Holbein forgery, called A Man with a Skull, as well as a painting that was originally acquired by the gallery as a work by Rembrandt, called An Old Man in an Armchair. It was signed and dated falsely and it was subsequently noted that the "loose treatment of the beard, fur coat and right hand, was weak" and could not have been created by the hand of the great painter. The gallery concluded that the painting instead, "displays a debt to Venetian, and in particular Tintorettesque portraiture" and is now attributed to an unidentified pupil or early follower of Rembrandt.

Another interloper in the gallery's collection is a fake Italian Renaissance portrait created in the early 20th century. The National bought it in 1923 as a 15th century work; the deception was exposed when analysis revealed it was created using pigments not available before the 19th century and that the surface has been coated to simulate the appearance of age. When compared with authentic Italian portraits of the late 15th century, numerous discrepancies also emerged. The painting shows a coat of arms of the Montefeltro family so it was assumed that it was a depiction of Federigo da Montefeltro, the Duke of Urbino, with two of his children. But when compared with actual portraits of the Duke, the face in the fake painting bears no resemblance to Federico, who had a large gash on his nose – an injury he famously received in battle.

Another work produced in the workshop of Andrea del Verrocchio, (where the likes of Botticelli worked as one of his assistants) called The Virgin and Child with Two Angels, has since turned out to be the work of at least two artists in the studio, due to the differences discerned in its painting style.

Dr Penny said the gallery would also stage a show featuring the Venetian painter, Canaletto. It would also allow mobile phone users to download 250 paintings as part of a digital initiative .