text: Britain's leading art gallery has uncovered new evidence in its attempt to keep a £1million Constable masterpiece that was looted by the Nazis from its Jewish owner during the Second World War. Tate Britain chiefs believe the chance discovery of an export permit will bolster their claim that the 1824 oil painting – Beaching A Boat, Brighton – was legitimately brought to Britain. The 1946 document bears the signature of a dealer called Karola Fabri and seeks permission for the transfer of artworks from Budapest to Zurich, including one by Constable identified as Fishing Boat. Scroll down for video . New evidence? Tate Britain chiefs believe the chance discovery of a 1946 export permit (pictured) will bolster their claim that Constable's painting was legitimately brought to Britain . Constable's masterpiece Beaching A Boat, Brighton was looted by the Nazis from its Jewish owner during the Second World War . The discovery of the permit in the archives of Budapest’s Museum of Fine Arts is the latest twist in an increasingly fraught dispute between the Tate and the descendants of the painting’s original owner, Baron Ferenc Hatvany, who died in 1958. The descendants of Hatvany – believed to be two daughters and a grandchild – say the Nazis stole the painting and smuggled it out of Hungary. It turned up in Britain in 1962 and passed through several hands before being donated to the Tate in 1986. The family launched a claim three years ago. The Government-backed Spoliation Advisory Panel, which rules on disputes over looted art, supported the heirs last year and said the Tate had a ‘moral obligation’ to return the painting. Sources close to the case say the export permit could prove the Tate’s claim that Hatvany voluntarily disposed of the painting while he was still alive. The Tate does not dispute that the painting was looted by the Nazis but the gallery’s understanding is that it was returned to the owner after the war and then legitimately exported. Stormy seas: The battle over Constable's painting evokes the story of a Jewish refugee's fight to reclaim a looted picture in the film Woman in Gold (pictured) Gallery chiefs are expected to argue that the permit proves the work was still in Hungary after the war. But The Mail on Sunday understands the claimants will argue the permit doesn’t change anything as it does not identify Hatvany. One source said: ‘If you had stolen the painting, you would still need to get an export permit to get it out of the country.’ A source at the Tate explained that there was legal duty to protect any assets owned by the British public, which is why the gallery argued that the painting should remain in the UK. They said: ‘No one disputes that the picture was at one stage looted by the Nazis, what is in dispute is what happened after it was returned to its owner.’ The dispute evokes the story of Dame Helen Mirren’s new film, Woman In Gold, which charts the story of how Jewish refugee Maria Altmann battled for almost a decade with the Austrian government to reclaim Gustav Klimt’s iconic picture of her aunt called Portrait Of Adele Bloch-Bauer I.
target: 'Beaching A Boat, Brighton' taken from Jewish owner in Second World War .
  Export permit discovery could bolster claims it came to Britain legitimately .
  Tate and descendants of painting's original owner are locked in dispute .