The carved oak door that led to the famous Friday Street Club was discovered in an auction in Nottingham by a German architect, Ulrich Hausmann. Mr Hausmann, an expert in antiques, paid only pounds 300, but now believes its true worth to be nearer pounds 500,000.
If the door is really part of the original frontage of the Mermaid Tavern, where the club used to meet, it must somehow have survived the Great Fire of London before being removed from the ruins of the venue in Bread Street, off Cheapside in the City of London.
The engravings on either side of a central panel on the door depict the playwright Ben Jonson and the academic William Camden.
So far, experts at British institutions such as the Museum of London and the Victoria and Albert Museum have been sceptical about the provenance of the door. They believe it is more likely to have been carved in the 18th or 19th century during a period of romantic nostalgia. If this is the case the door's value may be closer to pounds 10,000.
But Christopher Gilbert, the former director of the Leeds Art Galleries and Britain's top authority on antique furniture, believes Hausmann's claim to be genuine. "In nine out of 10 such cases, the object is likely to be a rogue," he said. "But this is the one in 10 that is right."
Specialists from the Victoria and Albert Museum first examined the door in 1990 but concluded that the carvings of Jonson and Camden were not 17th century.
Intriguingly, however, Jonson is not shown wearing the laurel wreath with which artists commonly anointed him after his appointment as Poet Laureate in 1616.
If Mr Hausmann fails to sell the door to a British institution he has threatened to auction it in New York.
Geraldine Norman, Sunday Review