Though the conservator who helped to reveal the painting's true identity called it "the rarest thing imaginable, " speculation is rife that there are other Leonardo da Vinci paintings still at large, possibly lying unknown in private collections.

CNN: Are there more lost Leonardo paintings out there?

The identity of the painting's subject is not known, but it is thought the balding, bearded man in a stiff, starched collar may be Juan Mateos, King Philip IV of Spain's master of the hunt.

CNN: Previously unknown Velazquez portrait discovered

The re-working was said to have followed a dismissive comment by a visitor to Wallington who called the work "a painting of a hat and ruffles" - but the identity of the real artist is still open to speculation.

BBC: Thomas Gainsborough portrait 're-worked by rival'