The bomb - which measures 5 foot in length - was found in Bermondsey, less than half a mile from the mainline rail from South East England into London Bridge station.
Today an expert army disposal squad will attempt to remove the 550lb bomb safely - but, if it does go off, it will obliterate buildings and homes up to 650ft away.
For this reason, some 1,200 households were ordered to leave their homes by 8am today.
A London Fire Brigade flyer to residents said: "The Army disposal team have advised that if the bomb explodes, buildings in the 200-metre zone will be significantly damaged and those close to the bomb will be destroyed.
"Remaining in your home is placing your life at significant risk. This course of action is not advised."
Those without jobs to go to are being housed in Southwark Council's Seven Islands Leisure Centre - and they may even have to sleep there.
It is hoped that the bomb will have been diffused by sundown, although it remains unclear as to whether this will be the case.
A Southwark Council spokesman said: "We can’t confirm a return time for residents as it all depends on the progress of the MOD down on the site.
"They plan to complete the work in daylight hours so we would hope that residents can return today, but clearly that could change as the day progresses."
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