Art experts have criticised plans by the National Gallery to move a fragile drawing by Leonardo da Vinci after it emerged that the institution's own conservators have warned that the work will never be fit to travel.

Last month, the gallery announced plans to loan the Burlington House Cartoon to the Louvre in Paris, as part of an exchange.

The first warning about the frail state of the 510-year-old drawing came in 1964. An internal report said the gallery could never "envisage the possibility of strengthening the support to such an extent that the Cartoon will ever be fit to travel".

Attempts to restore the work more than 20 years later were aborted because of the risks involved. In July 1987, a visitor fired a sawn-off shotgun at the drawing from seven feet away, creating a 150mm hole.

The loan appears to contradict the views of Sir Nicholas Penny, the National Gallery's director. In an interview with The Independent in February, he said the gallery was "more cautious than other institutions about loaning out work", and that risks should not be taken.

Michael Daley, director of ArtWatch UK, said: "We deceive ourselves if we believe that modern, scientifically assisted restorers can make good any injury that might arise. They cannot, as the best of them will admit."

The Louvre's version of Leonardo's Virgin of the Rocks will travel to London in November to hang in the National's exhibition "Leonardo Da Vinci, Painter at the Court of Milan". The Burlington House Cartoon will travel to Paris several months later.

A National Gallery spokeswoman said she was confident the work was fit to travel: "The National Gallery has given great consideration before agreeing to this exceptional loan. This collaboration between the two institutions has also taken into consideration the serious scholarly ambitions of the Louvre's exhibition, the research opportunity it provides and the interest to the public."