One of the world's finest paintings might not survive the journey to London for the National Gallery's blockbuster Leonardo da Vinci exhibition next year, according to a group of Polish experts fighting to keep the Renaissance masterpiece at home.

Historians and restorers in Krakow, home of the priceless Lady with an Ermine, say the National's bid to borrow the painting poses "a serious, even reprehensible, threat" to its existence. They claim the treasured panel depicting Cecilia Gallerani, mistress of Ludovico Sforza, ruler of Milan and patron of Leonardo, is too delicate to travel. It is one of barely 20 paintings attributed to the artist.

The Krakow division of the Association of Art Historians has sent an impassioned letter to ArtWatch UK, the restoration watchdog, to draw the world's attention to their "anxiety".

In a letter to be published tomorrow, its president, Joanna Daranowska-Lukaszewska, writes: "The sending of the Krakow picture abroad is insured for €300m [£250m], but in the event of a disaster no one would be able to restore this priceless masterpiece to life. We appeal to [ArtWatch] to draw the attention of world public opinion to the great dangers now faced by … common property of the whole cultural world."

It is not just in Poland that fears are growing over the National Gallery's plans for its exhibition, "Leonardo da Vinci: Painter at the Court of Milan". Concern is being voiced that other Leonardo paintings – La Belle Ferronière, from the Louvre; the Madonna Litta, from the Hermitage in St Petersburg; and Saint Jerome, from the Pinacoteca Vaticana in Rome – also face perilous journeys.

But in Krakow – where Lady with an Ermine sits in the Czartoryski Museum – those concerns have reached an acute level. "If you transport a picture panel such as the Lady with an Ermine … even the most ideal methods in the form of modern environmental chambers or special anti-shock frames are not able to sufficiently protect the work against a variety of vibrations, shocks or changes in pressure," said Professor Grazyna Korpal, dean of conservation.

"By allowing the painting to travel we create yet another serious threat [from] … possible human error. Such masterpieces require exceptional protection. The fundamental principle is the unconditional restriction of movement."

A source at the National Museum revealed that a letter from Poland's deputy cultural and national heritage minister permitted the picture to be transported to London.

Michael Daley of ArtWatch UK said it was hard to justify allowing such great masterpieces to be moved: "With paintings on panel, any movement is dangerous and bound to have a deleterious effect. Why take the chance? There is nothing to contradict the warnings of the Polish restorers. Moving the Lady with an Ermine is absolutely crazy."

He added that the danger of theft is heightened during any move. Last week a Picasso was among some 30 artworks valued at £4.1m stolen in transit between galleries in Germany and Spain.

Even moving works from room to room within a gallery presented dangers, Daley said. Two years ago, a Renaissance painting by Domenico Beccafumi was dropped while being taken off a wall at the National Gallery. Previously unpublished photographs of the damage will be published on ArtWatch's website tomorrow, along with the appeal from Poland and accounts of other accidents.

Adam Zamoyski, UK historian and chairman of the Czartoryski Foundation, confirmed that Lady with an Ermine had an export licence. He said: "It will be more closely guarded when in transit or on loan than when it is at its … home museum, which is now being rebuilt."