The subject of the portrait – believed to be worth hundreds of thousands of pounds – was a relative of Saskia van Uylenburgh, whom the artist wed in 1634. Dr Tico Seifert, senior curator for northern European art, said his curiosity was immediately piqued when he came across the etching in a box of prints because all known copies of the print are in reverse, unlike this one, which was nestled among dozens of copies of the artist’s work.

Further research, including authenticity checks with Rembrandt experts in Amsterdam, found the portrait of Sylvius was the only existing impression of the work in red ink.

Dr Seifert said: “These kind of plates were created through a chemical process which would see the artist polish the plate, cover it with a varnish and then take a very sharp, fine needle and scratch the varnish.

“The plate would then be put into an acid bath, so it was a chemical process rather than a mechanical process. You would take a wet piece of paper, put it on top of the plate and run it through a roller press. The big difference with this work is that it was printed in red ink. When I contacted colleagues in Amsterdam to find out about impressions in red ink, which are generally very rare, to my great surprise and delight they told me that this was a unique print.