“I can tell you, it nearly blew my socks off,” Johnny Van Haeften laughed. “In my – what is it? – almost 44 years in the art world I’ve never known an experience like this.”

Van Haeften, one of London’s most respected dealers in Old Masters, sounded almost breathless as he told his story. Speaking from his Mayfair gallery, he was describing his first sight of a painting by Pieter Brueghel the Younger that has for the 400 years since its creation been completely unknown to the world.

It has been continuously in the possession of the same English family since a distant ancestor bought it, direct from the artist’s studio, in Antwerp in 1611.

The owners had never publicised their precious possession, although they were well aware of its pedigree. Until the 1950s they even had among the family papers the original receipt, Van Haeften tells me, made out in Antwerp in 1611 for the purchase price of 200 florints. (If that mislaid piece of paper ever comes to light, it will in itself be a fascinating object for art historians.)