Excavations on a Tabard Street building site have unearthed a 2,000-year-old plaque showing the oldest example known of the Roman name for London.
The piece of stone, which measures 12 inches by 16 inches, is cut with the Latin letters Londiniensium and has been dated to between 50 and 150 AD.
The discovery was made on the building site for the Berkley Homes development in Tabard Street.
Experts say it is one of the UK's most significant archaeological finds for 20 years.
Archaeology experts from the building consultancy EC Harris examined the plaque and said it was possible that it refers to London's earliest champagne importer.
Nansi Rosenborg, senior archaeological consultant with the firm, said: "The inscription shows London's significance as a trading centre. The plaque may have been an advertising sign stuck in the side of the building.
"It relates to the French Champagne region of Rheims.
"Experts are arguing over the text but some say City slickers could have been drinking bubbly here two millennia ago."
A translation of the inscription reads: "To the spirits of the emperors (and) the God Mars Camulos, Tiberinius Celerianus, ranking moritex of the (traders) of London (set this up)."
Experts said that God Camulos appears to have been especially worshipped by people from the Champagne region of France, and they think the word moritex means negotiator.
This would seem to make the plaque a dedication to the gods from Tiberinius Celerianus, the chief trade negotiator from the area, based in London.
Gary Brown of Pre-Construct Archaeology which is carrying out the major dig on the one hectare site said the plaque was found in a pit near the remains of two large Roman brick buildings.
"The buildings could have been trading guild houses or even villas. We just don't know yet," he said.
Brown, who is just six weeks into a 40-week dig on the site, said Southwark had been a bustling commercial centre.
"I can't stress how important this site is. We have already gone back to the pre-historic occupation of the site and we have found vast quantities of artifacts," Brown said.
"We have so far only dug 15 percent of the site and we have already found this plaque, so the potential for more staggering finds is there. Who knows what more we will find?"