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The print dealer, who wishes to remain anonymous, made the discovery at Newark, the biggest mixed fair of its kind in the country with up to 4000 exhibitors.

The plate was covered with a thick layer of grime in a box under the table of a stallholder from Belgium or Holland. Once cleaned, it was revealed as an etching plate by van Leyden (1489-1533), an important Dutch artist.

The large engraving (111/2 x 163/4 in (29 x 43cm) was signed and dated L 1517. A crucifixion scene entitled Golgotha, the plate now has an estimate of £10,000-15,000 at Christie’s on June 22.

The dealer said: “I was driving back from the north and only stopped off at the fair to buy a few bits and bobs for the house. I was walking around and saw some intaglio printing equipment.

“I’ve been dealing in prints all my life and went over to have a look and asked if there were any more. They pulled out this box from under the table and I spotted this plate at the back.

“All the plates I’ve dealt with have been machined and rolled out but this one had a different feel of being hammered out by hand. At first I thought it was a plaque or name plate.

“It was covered in really thick grime but I rubbed away at a corner and realised it was an etching plate. I asked how much and he said £25. I bought it with other bits and pieces for £30.”

Once back in London the specialist cleaned the plate, made some prints and took them round to the British Museum.

Anthony Griffin, senior specialist at the time, kept the plate for a week and then identified it as the work of van Leyden.

Richard Lloyd, the specialist at Christie’s, said the engraving was one of only three large compositions by the artist and had long been considered one of his masterpieces.

Van Leyden, a contemporary of Albrecht Dürer, completed 174 engravings in his lifetime but only a set of 14 survive and are in the Antwerp Museum. The new discovery takes the total to 15.

Mr Lloyd said: “All the others have disappeared so this makes it rare and the fact that it is one of his large plates also makes it significant. Its survival is quite remarkable.”

Copper was a valuable commodity at the time and plates were often melted down for alternative uses or re-worked to produce further images. Leyden engraved his plates in a very shallow manner and he said the image had been re-covered by an unknown hand to “strengthen the lines”.