The great puzzle of what Lady Jane Grey, England's shortest-reigning monarch, looked like has been cracked. A little improbably, the answer may have been sitting in a house in south London for the last 100 years.

A likeness of Lady Jane has proved as elusive as a likeness of Shakespeare but Christopher Foley, a prominent London art dealer who has spent months researching the Streatham portrait, said yesterday that he was "very certain" that it was a contemporary image of the queen.

Lady Jane, who ruled for nine days in 1553 aged just 16, is the only English monarch since 1500 of whom no known portrait exists.

Experts at the National Portrait Gallery have agreed to study the picture and Mr Foley's evidence and if they confirm his attribution it is likely to be hailed as one of the most exciting English art discoveries for many years, even though it is by an unknown hand.