Roads were closed from the Victorian landmark and residents were escorted out after builders unearthed the unexploded bomb this morning.
Two schools in the area were also evacuated.
"Police officers remain at The Grange in Southwark, where a suspected unexploded WWII bomb was discovered by builders this morning," a Scotland Yard statement said.
The bomb discovered is also approximately 5 foot-long.
"A cordon and a wider exclusion zone of 400 metres has been implemented as a precaution, whilst we deal with the incident.
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A bomb squad was sent to the scene which falls in Bermondsey area of the city, south of the River Thames.
More than 50 people were initially evacuated from nearby homes and office buildings and it is now believed hundreds might have to stay in rest centres overnight.
All residents within 650 feet of the site are barred from returning to their homes. Instead, they will have to stay in "rest centres" at a leisure centre and library in the nearby area of Canada Water, Southwark Council said.
South London was one of the areas worst hit by the Blitz, which completely destroyed huge parts of the British capital in the early 1940s.